PURPOSE: To decrease the occupied space and to reduce the cost by providing plural transmission circuits multiplexing a parallel electric signal, converting it into a serial electric signal and sending the signal, plural reception circuits converting the serial signal into a parallel electric signal and receiving the signal and a selection circuit selecting which of plural transmission reception circuits are to be combined and used.
CONSTITUTION: Four signals given from an SD terminal 10 are converted into a serial signal by a multiplexer circuit 1 and outputted to an SOUT terminal 12 from a transmission circuit 2. Moreover, the serial signal inputted from an SIN terminal 13 passes through a reception circuit 6, converted into four substantial parallel signals by a demultiplex circuit 5 and the result is outputted from an RD terminal 11. A function selection section 9 uses function selection signals 15, 16 to generate signals 17∼20 controlling switches 21 to 28. Thus, the electric signal multiplex circuit with less occupied space and low cost is obtained.