PURPOSE: To convert the constant current output characteristic which is the defect of a solar cell characteristic to the constant voltage output characteristic easy to be used by a load device.

CONSTITUTION: A solar cell 1 with an optical filter is added serially to a solar cell main array 2 and a feedback control loop is constituted of a controller 3 with the fluctuation of a voltage V as input and a driver 4. When a load 5 becomes a light load, IL is reduced and the V is increased. Then, the controller 3 and the driver 4 increase the light shielding amount of the optical filter and the optical input of the solar cell is decreased. Thus, the output voltage is operated so as to let the output voltage V stabilize by V-I characteristics. That is, for feedback loop control operations, the constant current characteristic of the solar battery cell can be equivalentlly converted to the constant voltage characteristic.