Marunouchi Line

The line was the second route of the Teito Rapid Transit Authority (Eidan), the body that had earlier completed the Ginza Line, Japan's first subway. Eidan adopted the same third-rail collection method used on the Ginza Line and had planned to electrify at 750 V DC, but because the new line would share substations with the Ginza Line — and because the Ginza Line's depots were badly short of capacity — it settled on the same 600 V DC as the Ginza Line. The route carried the official designation "Line 4."

The first section opened on 20 January 1954, between a temporary Ikebukuro terminus and Ochanomizu, a distance of 6.4 km. When it opened it was the fourth subway to be built in Japan, after Eidan's own Ginza Line and the Midōsuji and Yotsubashi lines of the Osaka Municipal Subway. The line then advanced south and west in a series of short stages: Ochanomizu to Awajichō in March 1956, Awajichō to Tokyo in July 1956, Tokyo to Nishi-Ginza (today's Ginza) in December 1957, Nishi-Ginza to Kasumigaseki in October 1958, and Kasumigaseki to a temporary Shinjuku station in March 1959.

West of Shinjuku the extensions were built and named separately. From 1960 the section beyond Shinjuku was designated the "Ogikubo Line." On 8 February 1961 two stretches opened together — Shinjuku to Shin-Nakano (3.0 km) on the through route and Nakano-sakaue to Nakano-fujimichō (1.9 km) on what would become the branch. The western extension continued with Shin-Nakano to Minami-Asagaya in November 1961 and Minami-Asagaya to Ogikubo on 23 January 1962, completing the Shinjuku–Ogikubo run; six-car operation began the same day.

The network was finished on 23 March 1962, when Nakano-fujimichō to Hōnanchō (1.3 km) opened and the whole Ogikubo Line was through. The Shinjuku–Hōnanchō spur, informally called the Hōnanchō branch, was at first operated as part of the Ogikubo Line; on 1 April 1972 the Ogikubo Line name was abolished and the whole route was unified under the name "Marunouchi Line."

The original rolling stock — the 300, 400, 500 and 900 series — entered service from the 1954 opening and ran until 1996; many of these cars were later sold and exported for use on Line B of the Buenos Aires Underground. Their replacements, the 02 series six-car trains, began service on 17 October 1988, and by 18 July 1996, when the last 500-series cars were withdrawn from the branch, the older stock had been entirely retired. The 02 series in turn began to be replaced by the 2000 series, which entered service on 23 February 2019; the three-car 02 sets on the Hōnanchō branch ran until September 2022.

The Eidan era ended on 1 April 2004, when the Teito Rapid Transit Authority was privatised and its lines, stations, rolling stock and facilities passed to the new Tokyo Metro Co., Ltd. The line was progressively automated: automatic train operation (ATO) began on the Ikebukuro–Ogikubo main line on 27 December 2008, one-man operation followed across that section on 28 March 2009, and ATO was extended to the three-car branch trains in May 2010. On 7 December 2024 a radio-based CBTC train-control system was brought into use across the whole line.

• 195420 January: the first section opens between a temporary Ikebukuro terminus and Ochanomizu (6.4 km) — the fourth subway built in Japan, after the Ginza Line and Osaka's Midōsuji and Yotsubashi lines; the 300-series cars enter service.
• 1956March: Ochanomizu–Awajichō (0.8 km) opens; July: Awajichō–Tokyo (1.4 km) opens.
• 1957December: Tokyo–Nishi-Ginza (now Ginza) (1.1 km) opens.
• 1958October: Nishi-Ginza (Ginza)–Kasumigaseki (1.1 km) opens.
• 1959March: Kasumigaseki–Shinjuku (temporary station) (5.8 km) opens.
• 196021 October: the section west of Shinjuku (part of Line 4) is designated the 'Ogikubo Line'.
• 19618 February: Shinjuku–Shin-Nakano (3.0 km) and Nakano-sakaue–Nakano-fujimichō (1.9 km) open together; 1 November: Shin-Nakano–Minami-Asagaya (3.1 km) opens.
• 196223 January: Minami-Asagaya–Ogikubo (1.5 km) opens, completing the Shinjuku–Ogikubo run; six-car operation begins. 23 March: Nakano-fujimichō–Hōnanchō (1.3 km) opens, completing the whole Ogikubo Line.
• 19721 April: the 'Ogikubo Line' name is abolished and the whole route is unified under the name 'Marunouchi Line'.
• 198817 October: the 02 series six-car trains enter revenue service, beginning the replacement of the original 300/400/500/900-series cars.
• 199618 July: the last 500-series cars are withdrawn from the branch, completing the retirement of the original stock; many older cars are later exported for use on Line B of the Buenos Aires Underground.
• 20041 April: the Teito Rapid Transit Authority (Eidan) is privatised; the line, stations, rolling stock and facilities pass to Tokyo Metro.
• 200827 December: automatic train operation (ATO) begins on the Ikebukuro–Ogikubo main line.
• 200928 March: one-man operation begins across the Ikebukuro–Ogikubo main line.
• 201923 February: the 2000 series enters revenue service, beginning the replacement of the 02 series.
• 20247 December: a radio-based CBTC train-control system is brought into use across the whole line.