PURPOSE: To perform sure detection by providing circuits that can detect rotating directions and rotating quantity surely from the signals of two phases outputted from a PG when a PG shaft moves back and forth slighly or when the PG is oscillated.

CONSTITUTION: A circuitIwhich detects the rotating direction of a PG by latching the state of the signal A at the leading edge of the signal B out of the digital signals A, B of two phases outputted from the PG is provided. A circuit II which sets one-shot pulses in synchronization with the trailing edge of the signal B is provided. A circuit III which XORs the rotating direction set output of the circuitIand the signal A is provided. A circuit V which ANDs the output of the circuit II and the output of the circuit III is provided. A circuit V which screens the output of the circuit IV by each rotating direction in accordance with the rotating direction output of the circuitIis provided.