A semiconductor strain sensor has a gauge forming region on a p-type substrate surrounded by a p-type isolation region that reaches the p-type substrate. The p-type substrate is etched so that the entire bottom surface of the gauge forming region is covered by the p-type substrate, and the p-type substrate or p-type isolation region is not exposed to the etched recess portion or isolation groove, each of which have a relatively high number of defects. Thus, leakage current at the PN junction can be decreased to decrease a variation in the potential of the gauge forming region.