PURPOSE: To decrease greatly the number of filters by forming the circuits which supply the contact signals of a door open/close switch on the same printed base board and therefore supplying the signals of contacts provided at the following stages of an input signal filter circuit to each of above-mentioned circuit with no effect of noises.

CONSTITUTION: A printed base board 13 is provided within a joint block 8, and this board 13 contains an input signal filter circuit 14, a multiplex signal transmission circuit 15, and control circuits including a lamp control circuit 16, a key- coupled power window circuit 17, an automatic lamp switch-off circuit 18, and the like. A terminal 12 connected to a door open/close switch 1 is connected to the board 13 via a bus bar part 11, and the contact signal is supplied to the circuit 14. The output of the circuit 14 is supplied to the multiplex signal transmission circuit 5, the lamp control circuit 16, the key-coupled power window circuit 17, and the automatic lamp switch-off circuit 18 respectively. The circuit 15 fetches the contact signal that passed through the circuit 14 and then transmits a half-open door alarm signal to a multiplex transmission line 21 as one of multiplex transmission information.