A semiconductor pressure sensor includes a semiconductor substrate having a diaphragm portion. A diaphragm formation region including the diaphragm portion is electrically insulated from a peripheral region therearound. Voltage is applied to the diaphragm formation region via a pad and a wire both formed on a surface of the semiconductor substrate, for fixing a potential of the diaphragm formation region when the sensor is put in an operating state. The fixed potential is set to be equal to or higher than a maximum potential of a gauge diffusion resistive layer formed in the diaphragm formation region. As a result, even when the maximum potential of the gauge diffusion resistive layer is a power supply voltage, it can be prevented that current leaks from the gauge diffusion resistive layer.