We deployed the JISNET broadband seismic network in west and central Indonesia. Using data from JISNET and other broadband stations, we estimate the thickness of the mantle transition zone beneath East and Southeast Asia and the Philippine Sea using the ScS reverberation method. The thickness is about 240 km beneath East and Southeast Asia, 280--290 km beneath the western edge of the Philippine Sea, 260--280 km beneath the central part of the Philippine Sea, and 260 km beneath the Izu-Bonin Arc. These results imply that the temperature is about 300 K lower than the global average beneath the western edge of Philippine Sea, 200 K lower beneath the mid part of the Philippine Sea, 130 K lower beneath the Izu-Bonin arc, and almost equal to the average beneath East and Southeast Asia.