PURPOSE: To protect a battery, by adding the second accessory terminal (An ACC terminal) to an existing ignition key switch, by connecting parts, which greatly consume power, to the ACC terminal and by supplying electric power only during the operation of an engine.

CONSTITUTION: A rotary ignition key switch 1 is further provided with the second ACC terminal 6 besides the first ACC terminal 3, an IG (Ignition) terminal 4 and a STA (Starter) terminal 5, and formed in four stages. When a key is inserted and connected to ACC contacts from OFF contacts, electric power is supplied only to the first ACC terminal 3 connected to a device that consumes power in a small amount. When an engine is operated by connecting the key to STA contacts and the key is beforehand connected to ON contacts, electric power is also fed to the second ACC terminal 6. Electric power is not supplied even to any ACC terminals during the time when the key is connected to the STA contacts.