When transferring signals from the input side to the output side, a photocoupler is a device capable of transferring them between circuits in a state of electrical isolation. Inside the photocoupler, electrical input signals are converted into optical signals once by using a light-emitting device, and then these optical signals are output after being reconverted into electrical signals by using a photodetector device. In many devices such as FA, OA, and home electronics devices, different DC and AC power supply systems are placed inside the same board, and signals are transferred between them. If these systems are directly connected, a safety problem may develop in terms of operation. By using a photocoupler, even in such a case, signals can be transferred in a state of electrical isolation.