A series hybrid vehicle comprises a generator 30 driven by an internal combustion engine 40, a battery 20 chargeable by generator 30, an electric motor 10 rotated by electric power of generator 30 and battery 20. A parallel hybrid vehicle comprises. a battery 200 chargeable by an electric motor 100, and selectively uses an internal combustion engine 400 and electric motor 100 as driving source for driving vehicle wheels 900. In these -hybrid vehicles, there is provided a sensor 21 or 201 for detecting the state of charge (SOC) of battery 20 or 200. An output of generator 30 or internal combustion engine 400 is controlled based on each the SOC and a variation the SOC.