Abstract: Disclosed is a corrosion preventing circuit for a switch which, even if a large current switch is used for a small current system such as an electronic unit, can improve long-term durability of the switch in a manner similar to the present level without increasing the burden on the system side, thereby ensuring the reliability of devices belonging to the system. A series circuit consisting of a switching means and a current limiting resistor is connected in parallel to a resistor that is connected in series between a dc power supply and a large current switch. A control circuit turns on the switching means by generating a pulse of a predetermined time width after a predetermined time in accordance with the turning on of the large current switch. The current flowing while the switching means is being turned on is applied to the large current switch, so that an oxide film formed at the contact of the switch is destroyed.