PURPOSE: To provide a pressure sensor which can output the small differential pressure fluctuation value from rated pressure, separately from the gauge pressure value of measure-target equipment, as the output of a sensor.

CONSTITUTION: A pressure detector 1 has a semiconductor distortion gauge 2, which detects the pressure of equipment being the subject of pressure measurement, and a voltage detecting circuit 3, which detects the output of a semiconductor distortion gauge 2 as the specified voltage value. A gauge voltage output part 4 has an output signal converting circuit 5 which converts the output from the pressure detector 1 into specified electric signal and output them. A small differential pressure output part 6 has a reference voltage generating circuit 8, which generates voltage equivalent to the rated voltage of the equipment being the subject of the pressure measurement, a subtracting circuit 7, which seeks the difference by subtracting the output of the pressure detector 1 from the output of the reference voltage generating circuit 8, an amplifying circuit 9, which amplifies the output of the subtracting circuit 7, and an output signal converting circuit 10, which converts the output of the amplifying circuit 9 into the same electric signal as the output of the gauge pressure output part 4.