To provide a semiconductor sensor wherein a diaphragm is hardly damaged, and dispersion of sensor sensitivity is small.

A recess 26 is formed on the under surface of an SOI substrate formed by laminating an Si substrate 22 on an Si thin film 24 through an SiO2 film 23. Part of the Si thin film 24 is a diaphragm 25 used as a pressure-sensitive domain. In an upper surface outer peripheral part of the recess 26, the SiO2 film 23 covers the under surface outer peripheral part of the diaphragm 25, and the under surface of the diaphragm 25 is exposed in a domain excluding the upper surface outer peripheral part of the recess 26. The SiO2 film 23 (reinforcement part 23a) covering the under surface outer peripheral part of the diaphragm 25 has a taper on the under surface, and the film thickness becomes thinner gradually from the outer peripheral part of the diaphragm 25 toward a center part of the diaphragm 25.