To ensure a constant relay function time at all times by inserting a delay circuit between a button and the relay coil of a relay circuit and delaying function of the relay circuit on the button operation side when the button is switched thereby preventing plugging and realizing a reliable switching operation.

Delay circuits 13, 14 comprises resistors 13a, 14a connected in series between the output terminals of bridge rectifier circuits 11, 12 connected with ascend and descend buttons 6, 7, and capacitors 13b, 14b connected in parallel. The delay circuits 13, 14 delay excitation of relay coils 15, 16 in a latching relay 8 by a time required for charging the capacitors 13b, 14b. Since the exciting timing of the relay coils 15, 16 can be delayed by a predetermined time, adverse effect of overcurrent can be eliminated at the time of plugging. Furthermore, plugging can be prevented by having a stabilized buffering time at all times.