PURPOSE: To obtain a power source controller in which an improvement of a fuel cost and prevention of overdischarge of a battery can be compatible.

CONSTITUTION: Power generation control means 301 supplies a power generation amount variable request to voltage regulating means 304 based on a detected output of a vehicle state detecting means 4. Charging rate detecting means 302 calculates a charging rate according to a charging current iB, and corrects it by a correction coefficient from correction coefficient generating means 306. The means 304 so decides a field current If according to the request from the means 301 and the rate from the means 302 that a terminal voltage VB of a battery 1 becomes a target voltage. Field current control means 305 drives Tr according to the current If decided by the means 304. The means 304 executes a decrease in the current If due to the request of the means 301 only when the rate is a predetermined value or more.