A piezoresistive sensor includes a piezoresistive region to which first conductivity type impurity has been introduced, the piezoresistive region being formed in a semiconductor layer; a protection region to which second conductivity type impurity has been introduced, the protection region covering a top of a region in which the piezoresistive region is formed, the protection region being formed in the semiconductor layer; and contact regions to which the first conductivity type impurities have been introduced, the contact regions being connected to the piezoresistive region, the contact regions being formed so as to reach a surface of the semiconductor layer except a region in which the protection region is formed, in which the following inequality holds: impurity concentration of the piezoresistive region