A counter within a rotary encoder outputs 8-bit count data CT for counting up at a period equal to or faster than the transmission period of positional data. When the rotary encoder receives a communication frame for positional data request command STX at a receiving circuit from a servo control apparatus, a sensor control computer feeds a positional detected information including a destination address data DA indicating the servo control apparatus, a source address data SA, 40-bit positional detected data PS, 8-bit status data STS, and count data CT to a transmitting circuit. The transmitting circuit adds a 16-bit cyclic code CRC allowing the detection of an error in the input data to the end of the input positional detected information, converts it to serial data as serial signal SRX, and transmits it to the servo control apparatus.