PURPOSE: To prevent a trouble due to the misresetting of a switch, by breaking the current path of a switch circuit to switch the voltage of high or low margin test when the initializing signal of a CPU is detected.

CONSTITUTION: For instance, the initializing signal obtained from initialization that is executed first is applied to actuate a CPU20 while the 1st switch 15 is kept at a position U. Thus a driving circuit 22 stops its working, and therefore the driving of the 3rd relay 21 is stopped. As a result, a relay switch 23 is opened and the 1st and 2nd relays 17 and 19 are neither driven at a time. Then a synthetic resistance of resistances 8, 11, 9 and 12 is inserted between an error detecting input of an error amplifier 2 and a ground. The stabilized output voltage 5V thus obtained is applied to each part of the CPU20 to perform an ordinal operation. When a margin test is carried out again, the 2nd switch 24 is turned on.