PURPOSE: To monitor a rotational frequency properly by providing a sampling signal generating circuit, supplying pulses to a series circuit of a sensor and a resistance and putting the sensor in sampling operation, and providing a battery as a power supply to an integrated circuit.

CONSTITUTION: A pulse generated for every turn of a rotating body 1 is utilized to counts up and down the rotational frequency of the rotating body 1, the battery 8 as a power supply to an integrated circuit is provided, and sensors 3 and 4 are put in operation with sampling pulses SLP from the sampling signal generating circuit 7. A code plate 8 provided on the rotating shaft of a motor and detectors 91 and 92 provided opposite it constitute the rotational angle detector 9 for ≤1 turn, and its output and the output of the counter circuit 5 are inputted to the output circuit 11 in parallel. Thus, the consumption of the battery is reduced and the economical, small-sized device is constituted.