Preventing corrosion of a contact circuit

A determined appropriately the progress of corrosion of the contacts, to provide a contact corrosion prevention circuit it is possible to prevent effective corrosion. An input terminal 11x to which an input signal line 140 is connected is used to connect to the contacts of the power supply low side. Since the contact is connected between the pull-up resistor 148 to the power source voltage VB side and the ground side, the potential of the input signal line 140, the contact becomes the potential of the power supply voltage VB side in the open state, contacts in the closed state, the ground-side potential. When the contact resistance contact is corroded increases, contact the potential of the input signal line 140 when the closed state increases. Comparison means 143 detects that the potential of the input signal line 140 becomes higher than a predetermined potential, the low impedance section 141 is activated, as it reduces the impedance of the pull-up resistor 148, current flowing from the power source voltage VB side in, it is possible to remove the corrosion and heat the contacts. .The