A diaphragm made of a polysilicon film is provided above a first main surface of a silicon substrate. Four gauge resistors are provided on the top face of the diaphragm. A through hole for exposing the bottom face of the diaphragm is formed in the silicon substrate. An anchor portion for mounting the diaphragm on the silicon substrate is provided between the diaphragm and the silicon substrate in a manner surrounding circumferentially an opening end of the through hole located at the side facing the first main surface. Accordingly, a semiconductor pressure sensor having a diaphragm of reduced and less varied thickness can be obtained.