Bathymetric and geophysical (magnetic and gravity) mapping were conducted in the Mariana Trough between 19｡15ﾕN and 22｡20ﾕN and the West Philippine Basin aroung the central Basin Fault (CBF) during R/V Kairei KR9812 cruise. Spreading ridges in the mapped area of the Mariana Trough consists of five segments. The segment boundary show typical structure of slow-spreading ridges, those are nodal deeps and inside-corner highs, except for the northernmost boundary. It is estimated from magnetic anomalies that seafloor spreading between 19｡ and 20｡N started at 5 Ma or a little older, and the average spreading half-rate during the last 3.6 m.y. is about 1 cm/year. The centrl part of the CBF between 127｡50ﾕE and 131｡E can be divided into four segments bounded by N-S trending fracture zones. Extinct spreading centers of E-W trend with parallel minor ridges, noda deeps and inside-corner highs were cut by lineaments and depression of NW-SE trend, These features can be interpreted as the combination of the earlier N-S spreading and the late NE-SW extension, although the age and cause of the latter remain unclear.