Tōhō Line

The line grew out of the same congestion problems that the Namboku Line had been built to relieve. Although Sapporo's populous Higashi-ku had been served by the Namboku Line since 1971, the main way into the city centre remained the bus, and heavy bus running combined with winter snow narrowing the roads produced chronic traffic jams; after the Namboku Line opened, feeding bus routes into its stations concentrated riders so heavily that peak-hour crowding on the Namboku Line exceeded 200 percent. To relieve this, the project for a new line — planned in 1971 as Subway Line 3, a "Second Namboku Line" — was pushed forward, led by city-assembly members elected from Higashi-ku.

The city assembly approved construction of Line 3 on 9 August 1980. A railway-laying licence for the Sakaemachi to Susukino (provisional name) section was obtained on 30 January 1982, and construction of that section began on 6 July 1983. On the southern end the line had originally been intended to extend toward the Yamahana district to replace the Yamahana streetcar lines, but local opposition to abolishing the streetcar, together with worsening congestion on National Route 36 in Toyohira-ku, redirected the route toward the Tsukisamu and Kitano areas instead.

The first segment, from Sakaemachi to Hōsui-Susukino (8.1 km), opened on 2 December 1988, with the new 7000 series trains entering service; it was the last new railway line in Japan to open in the Shōwa era. Because the route finally chosen through Higashi-ku was a compromise that ran via the Higashi-15-chōme Tonden road rather than the busier district-office corridor, early ridership fell well short of forecasts, and the line was for a time derided by residents as a "political line." On the southern side, weaker-than-expected population growth around Kitano had already led the city, on 1 September 1987, to shorten the planned extension to Fukuzumi — three stations short of Kitano.

Construction of the Hōsui-Susukino to Fukuzumi section began on 8 January 1990, and that 5.5-kilometre extension opened on 14 October 1994, completing the line to its present length; five additional 7000 series train sets (twenty cars) were built for it. To save on construction costs, the stations on the extension were all built with island platforms and only enough length for six-car trains, whereas the original Sakaemachi–Hōsui-Susukino stations had been built to take eight-car trains. In practice every Tōhō Line train has remained four cars long.

Over the following decades the line was steadily modernised. The SAPICA IC card was introduced on 30 January 2009. From 8 May 2015 a fleet of new 9000 series trains began replacing the 7000 series, which made its final run on 25 June 2016 — the Tōhō Line was the last Sapporo subway line still operating the cars it had opened with. Automatic train operation (ATO) using the 9000 series began on 18 July 2016, platform edge gates were completed at every station by 24 March 2017, and one-man (driver-only) operation began on 1 April 2017.

Today the Tōhō Line carries the fewest riders of Sapporo's three subway lines, though its ridership has trended upward; the English Wikipedia article records about 140,500 daily riders in 2023, roughly 94 percent of the 2019 figure. Its trains are based not on the line itself, which has no depot, but at the Nishi depot on the Tōzai Line, reached by a dedicated connecting track via Sapporo Station. In an unusual incident on the night of 8 September 2022, foxes that had wandered onto the tracks twice brought trains to a temporary halt — the first time wildlife had disrupted operations on the Sapporo Municipal Subway.

• 1971The line is included in Sapporo's long-term plan as Subway Line 3 (a 'Second Namboku Line'), proposed as an 11 km Motomachi–Yamahana route.
• 19809 August: the Sapporo city assembly approves construction of Line 3.
• 198230 January: a railway-laying licence is obtained for the Sakaemachi–Susukino (provisional name) section.
• 19836 July: construction begins on the Sakaemachi–Susukino (provisional name) section.
• 19871 September: the planned southern extension is shortened to terminate at Fukuzumi, three stations short of Kitano.
• 198828 September: a railway business licence is obtained for the Hōsui-Susukino–Fukuzumi section.
• 19882 December: the Sakaemachi–Hōsui-Susukino section (8.1 km) opens with 7000 series trains — the last new railway line to open in Japan in the Shōwa era.
• 19908 January: construction begins on the Hōsui-Susukino–Fukuzumi extension.
• 199414 October: the Hōsui-Susukino–Fukuzumi section (5.5 km) opens, completing the line; five more 7000 series sets (20 cars) are added.
• 200930 January: the SAPICA IC card is introduced.
• 20158 May: the new 9000 series trains begin service, starting the replacement of the 7000 series.
• 201625 June: the 7000 series makes its final run — the Tōhō Line was the last Sapporo subway line still running its opening-era cars. 18 July: ATO automatic operation with the 9000 series begins.
• 201724 March: platform edge gates are completed at all stations. 1 April: one-man (driver-only) operation begins.
• 20228 September: foxes that wandered onto the tracks twice halt trains overnight — the first time wildlife had disrupted Sapporo Municipal Subway operations.