Disclosed is a signal processing apparatus comprising a sensor whose impedance varies according to changes in quantity sensed thereby, a power source applying a voltage to the sensor, a current/voltage conversion circuit for converting an output of current flowing through the sensor to a voltage signal, and a feedback means for passing a current corresponding to the output voltage of the current/voltage conversion circuit through the sensor. In a practical example thereof, a photosensor as the sensor is supplied with the voltage, the output current of the photosensor is supplied to the input terminal of the current/voltage conversion circuit, the output voltage of the current/voltage conversion circuit is compared with a reference voltage in a comparison circuit, the output thereof is supplied through a rectifier circuit to a voltage holding circuit to be held thereby and also supplied to a buffer circuit, the output voltage of the buffer circuit is supplied through a resistor to the input terminal of the current/voltage conversion circuit, and a processed signal is output from the output terminal of the current/voltage conversion circuit.