PURPOSE: To transmit emergency stop signals simultaneously to all electric machines, even if there are some nonconducting electric machines in a group of electric machines, by turning any one emergency stop switch of each electric machine OFF thereby isolating a driving power supply line from a high potential side power supply line and turning all emergency stop signal transmitting means ON.

CONSTITUTION: When some electric machine Xj(j is 1, 2...n) is turned OFF by depressing an emergency stop switch Sj, a switch line ls is disconnected from a driving power supply line lD at the point of the emergency stop switch Sj, and thereby high potential on a high potential side power supply line lH is not applied on the driving power supply line lD thus de-energizing all relay coils R1-Rn. Consequently, relay contacts r1-rn are turned OFF to open all emergency stop input lines L1-Ln thus emergency stopping all electric machines X1-Xn. According to the constitution, emergency stop signals can be transmitted to all electric machines simultaneously even if there are some nonconducting electric machine in the group.