detected a detection signal is transmitted from the sensor section 9 to the sensor control section 8 and after it is subjected to signal analysis it is transmitted from the child station input output section 7 to the parent station 4 through the dp signal line 5 and the dn signal line 6 as a presence absence signal of the body to be detected 11 in fig6 two bodies to be detected 11 which are circular wafers and positioned on a lowermost stage are stored in a stacked state to each other and such an abnormal state is detected by a light reception signal shown in fig2 fig7 is a function block wiring diagram of the child station input output section 7 and the sensor control section 8 the child station input output section 7 transmits and receives signals transmitted on the dp signal line 5 and the dn signal line 6 on the other hand the child station input output section 7 receive an out signal 25 from mpu 20 so that a detection result of the body to be detected 11 which has been determined by the mpu 20 basing upon the detection signal from the sensor section 9 is transmitted to the parent station 4 the child station input output section.