PURPOSE: To make it possible to transmit and receive a noncontact (for instance, photocoupling) signal with high reliability, by providing an error detection means.

CONSTITUTION: A data 108 from the transmission side device 100 is received via the photodetector 112 by the photodetector 221 provided on the receiving side device 200, and a data 209 can be obtained. The data 209 is sent back again to the photodetector 141 of the transmission side device 100 through the light emitting element 232. And the data 108 sent from the transmission side by the EOR circuit 151 is made to coincide with the data 209 sent back from the receiving side, and a coincidence signal 110 is generated. The coincidence signal 110 is sent to the receiving side device 200 by the light emitting element 162 and the photodetector 271. And the receiving side device 200 accepts the data 209 which has been received at first, by regarding the data as correct while the coincidence signal 211 is obtained.