PURPOSE: To realize the use of a logical operation circuit even with a power supply of high voltage where a logical IC is not available, by connecting the photodetecting parts of photocouplers in series with the current flowing directions set equal to each other and outputting a prescribed logical level when a current flows to the train of said photodetecting parts.

CONSTITUTION: When the outputs of signal sources 1, 2 and 3 are equal to logic '1' photocouplers 4, 5 and 6 are turned on and a current flows to resistances 7 and 8 and the base of a TR10. Thus the TR10 is turned on to supply a collector current. When this collector current is large enough, the TR10 is saturated and turned on. Thus the voltage of an output terminal 11 is kept at about 48V. If either one of outputs of sources 1∼3 is equal to logic '0', the photocoupler connected to said output is turned off and no current flows to both resistances 7 and 8 and the base of the TR10. As a result, the TR10 is turned off and supplies no collector current. Therefore, the voltage of the terminal 11 is approximately equal to a ground potential to act in the same way as a NAND gate of a logical IC.