To provide a signal processing circuit which performs waveform shaping at an optional bias point of the top and bottom of a signal that is outputted from an object to be detected and also to provide a signal processing circuit board and an IC which are used for different kinds of sensors in common.

This device consists of a signal amplifier circuit 2 and normal rotational and inverted comparator circuits 4 and 3 which perform waveform shaping. The circuits 3 and 4 are provided with auxiliary bias circuits 5 and 6 respectively in order to set a hysteresis characteristic whose top and bottom are almost equal to an input waveform and top and bottom threshold potentials to the center of a signal. Also, a single setting element and a common variable element 7 which function the circuits 5 and 6 in common are provided. A constant which is adapted to an operation that is optimum in an environment where a sensor is installed by the single setting element or a common variable element 7 or a different kind of a sensor is set. Also, this signal processing circuit changes a constant of a fixed resistor on a substrate and is common to different kinds of sensors with an exterior resistor of an IC.