In this paper, we discuss the diurnal variation of the air-pressure system in the urban Tokyo in calm summer days by using the METROS (Metropolitan Environmental Temperature and Rainfall Observation System) data. The results are summerized as follows:
1) Throughout almost one day, the sea-level pressure is low in the northern and southern sides of the urban Tokyo, and high in the western and eastern sides.
2) The sea-level pressure difference inside the urban Tokyo is assumed about 0.8-1.3hPa.
3) During the daytime when the surface air-pressure is low, the pressure drop in the morning is larger in the suburban Tokyo, and the pressure drop in the afternoon is larger in the downtown Tokyo.
4) After sunset, firstly the pressure rise is larger in the downtown Tokyo, thereafter in the suburban Tokyo.
5) Not only surplus heat, but also semi-diurnal atmospheric tides and local cyclones generated in the mountainous area of central Japan cause the diurnal variation of the air-pressure system in the urban Tokyo.

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