PURPOSE: To enable utilizing a power source such as a solar battery, wave activated power generation device and a wind power generation device which, for itself, is unable to continuously supply protection currents over a long period of time as a power source for corrosion protection without using a battery.

CONSTITUTION: Change of potential to ground in the lapse of time after the shutdown of a protection current to an object to be corrosion-protected 1 is measured. Change of a protection current density to the object having slow change of the potential or the object in the lapse of time is measured. Electric protection to the objects are executed with supply of the protection current from a power source such as the solar battery 3 which is able to only intermittently supply the protection current to the object having larger drop of protection current density than a designed value. In such a manner, a battery and a charge-discharge controller, etc., are unnecessary and the system is simple to reduce the cost. Further, maintenance operation such as a periodic exchange of a battery for a new one is unnecessary and maintenance cost is reduced, too.