To provide a controller of a generator for a vehicle and a power supply system for a vehicle capable of reducing power generation loss and the capacity of an excitation power supply.

A generator 1 for a vehicle comprises a stator coil 2, a rectifier 3, a field coil 4, and a voltage controller 6. The voltage controller 6 is provided with power transistors 61 and 62 which are turned on at such timing as an excitation current is supplied to the field coil 4 from a battery 9, and circumfluence diodes 63 and 64 which regenerate magnetic energy of the field coil 4 on the battery 9 at such timing as the power transistors 61 and 62 are turned off.