PURPOSE: To obtain a rear defogger device for automobile, of high reliability at low cost, without installing a special sensor and the like by electrifying the rear defogger only for a previous period of time passed, which is stored in a memory, when a switch is turned ON at a certain point.

CONSTITUTION: A fixed control program is stored in a control unit 1, and the ON/OFF condition of a rear defogger 3 is appropriately controlled. Namely, a relay contact RS of a relay 2 installed in the side of a power source (IG1) of the rear defogger 3 is controlled ON/OFF through a relay coil RL by the switching of an output side built-in transistor Q1. The time passed from when a rear defogger switch 4 is once turned ON, to it is then turned OFF, is stored in a built-in memory (RAM) 5 of the rear defogger control unit 1, and is thus updated. At the time, when the switch 4 is turned ON at a point of time, the rear defogger 3 is electrified only for a previous period of time passed, which is stored in the memory.