the selector switch 45 is set at auto the parallel running between the generators 60 61 is done automatically according to the quantity of the load in the embodiment the frequency voltage and phase of the generator 60 and or the generator 61 are adjusted by manual operation since the electric power 11 from shore is supplied to the cargo handling equipment 63 which requires a large capacity of electric power either one of the generators 60 61 is enough to supply electric power to the ordinary load 67 after the changeover in the second switchboard 33 only the generator 61 is running in the embodiment electric power is supplied from the generator 61 to the main bus bar 62 with the circuit breaker 73 meanwhile the electric power 11 from shore is supplied to the power line 66 through the first line 13 in a state that the circuit breaker 70 is open the voltage frequency and phase of the main bus bar 62 and those of the electric power 11 from shore are synchronized using the voltmeters 23 the frequency meter 24 and the synchronizer 25 the first step immediately after the first step the circuit breaker 70 is closed to connect the power line 66 to the main bus bar 62 in this instance the electric power 11 from shore is transmitted to the main switchboard 75 when the ship is supplied with electric power only from the generators the electric power is transmitted from the main switchboard 75 to the propulsion auxiliary equipment 65 via the power line 66 however in the case of the present invention the electric power 11.