A receiving unit converts a pulse received from a transmission unit into a digital output signal and send back the received pulse to the transmission unit via an isolated communication unit, when it receives the pulse transmitted from the transmission unit; the transmission unit includes: an edge pulse generating circuit for generating a pulse corresponding to a transition of the digital input signal between a first level and a second level; and a verification circuit configured to receive the pulse sent back from the receiving unit, determine whether or not the pulse has been sent back from the receiving unit, and generate an additional pulse when the verification circuit determines that the pulse has not been sent back from the receiving unit and is configured to transmit the pulses generated in the edge pulse generating circuit and in the verification circuit, to the receiving unit, via the isolated communication unit.