PURPOSE: To enable the changeover time of an other system input power source to be set to an enough long time by making a power voltage monitor into one fitted with a delay operation contact and an electromagnetic contact into a mechanical latch type.

CONSTITUTION: A power voltage monitor 3 is equipped with an operation coil 4, an ON delay operation a contact 5a, an OFF delay operation a contact 5b, and a b contact 5c of the same operation. One electromagnetic contactor 6 is made a mechanical latch type equipped with a making coil 7, a trip coil 8, a main contact 9, an auxiliary b contact 10a, and an auxiliary a contact 10b, and the other electromagnetic contactor 11 also is a mechanical latch type of the same constitution as the electromagnetic contactor 6. And, the making coil 7 is connected to one input power source 1a through the contact 5a, and the trip coil 8 to the input power source 2a through the contact 5c, and the trip coil 13 to the input power source 1a through the contact 5b, whereby the instantaneous stop time at power source changeover or the close holding time of the main contact 9 at load short-circuit occurrence can be made enough long.