PURPOSE: To transmit a large amount of data while varying the amount of transmitted information by providing a storage circuit, which corresponds to the amount of transmitted information, on a receiver side.

CONSTITUTION: On the basis of the timing of a clock circuit 61, a counter circuit 62 operates. The contents of the counter circuit 62 are decoded by a decoding circuit 63. According to the decoding result, one of driving circuits 64 is selected. The selected driving circuit 64 transfers on-off information on an optional group of contacts 4 and an answer signal SA to a level converting circuit 11. After sending out a synchronizing signal S1, a parallel-serial converting circuit 12 sends out the count value of the counter circuit 62 as an address information signal S3. Then, the external answer signal SA and a data signal S2' consisting of pieces of on-off information on respective contacts are sent out in order. A receiving circuit 21 receives the signals from a transmission line 3 and sends out them to a serial-parallel converting circuit 22, an address discriminating circuit 71, and a timing control circuit 74. The data signal S2' to be stored is sent to a storage circuit 72, where it is stored.