PURPOSE: To decrease the number of parts and to reduce cost by forming an index signal from a magnetic pattern for switching exciting phase on a rotor.

CONSTITUTION: A counting circuit is constructed so as to turn an output 50 of a counting circuit 5 into one pulse at every rotation of a rotor 1, a one-shot circuit 6 is triggered at the rise of the output 50, and one index signal 7 is prepared at every rotation of the rotor 1. That is to say, an output 40 in accordance with the magnetic pattern is inputted from a hole element in a hole sensor 4 (or 2 or 3) to the counting circuit 5. Condequently, the index signal 7 can be obtained without providing a magnet on the rotor 1 or providing the hole element to detect the magnet. Thus, the number of parts can be decreased and the cost can be reduced.