The anomalies of geomagnetic total force over Japan and its surrounding area (20¿--50¿ N and 120¿--160¿ E) were interpreted by a magnetization of the earth's surface layer in a direction parallel to the main magnetic field. A negative magnetization anomaly in the Japan Sea may be attributable to the thinness of the magnetized part of the crust in comparison with the Japan Island. This is in accordance with the underground high temperature gradient suggested from the observed high heat values in the Japan Sea area.