The air temperature distributions along the Hibiya Line were observed 86 times by the train traverse observation method with a thermister thermometer from 26 April 1987 to 1 January 1991. Two observation methods were used. One was to observe air temperatures every 10 seconds to determine the temperature distribution along the subway line. The other was to read the air temperature when the train reached the platform, and use that value to represent the air temperature on the platform. The results obtained are summarized as follows; (1) Air temperatures on the platforms are higher than those in tunnels. (2) Air temperatures at Hibiya station and Ginza station are higher than at other stations in seasons when air-conditioning is not required. Air temperatures on air-conditioned platforms and tunnels are relatively low in the season when air-conditioning is required. (3) The air temperatures around the subway platforms have smaller diurnal variation than outside air temperatures. (4) Air temperatures rise in proportion to the outside air temperatures, with small gradients. (5) The gradients of regression of air temperatures on the platform to outside air temperatures range from 0.35 to 0.55. (6) In most stations the equilibrium temperature T*, at which air temperatures on platforms are equal to the outside air temperatures, is higher than the warmest monthly average normal air temperature (26.7°C) at Otemachi, Tokyo.