A semiconductor device manufacturing method is provided including: forming a first impurity layer that becomes first wells in a high breakdown voltage transistor forming region in a semiconductor layer; forming a second impurity layer that becomes offset regions in the high breakdown voltage transistor forming region; forming the first wells and the offset regions by diffusing impurities of the first and second impurity layers by heat treating the semiconductor layer; forming element isolation regions by a trench element isolation method in the semiconductor layer, after forming the first wells and the offset regions; forming first gate dielectric layers in the high breakdown voltage transistor forming region; forming second wells in a low voltage driving transistor forming region in the semiconductor layer; forming second gate dielectric layers in the low voltage driving transistor forming region; and forming gate electrodes in the high breakdown voltage transistor forming region and the low voltage driving transistor forming region.