An apparatus for controlling power generation of a generator mounted on a vehicle, the generator charging a on-vehicle battery, comprising a detecting device that detects information indicating the internal status of the battery that includes temperature of the battery, a power supply circuit that supplies power to the detecting device, a calculator for calculating the internal status of the battery using the information detected by the detecting device, a circuit board on which the calculator, the power supply circuit, and the detecting circuit are mounted and a controller that controls the power generation of the generator based on the internal status of the battery. The temperature sensing element is arranged on a bus bar that is electrically connected to the negative terminal of the battery, and the bus bar and the temperature sensing element are coupled thermally to each other.