PURPOSE: To reduce the fuel consumption by accumulating the gas which is used fruitlessly by controlling the number of revolution of a turbine according to the signal from a boiler automatic combustion controller.

CONSTITUTION: A steam control valve 7 is ON/OFF-controlled by the signal C from a boiler automatic combustion controller 10, and the number of revolution of a turbine 2 is adjusted to control the amount of gas fed into a boiler 4. Each of upper and lower limits of the allowable pressure in a cargo tank 1 is set by the signal A transmitted to the boiler automatic combustion controller 10 from a pressure controller 5. Therefore, when the surplus steam is generated, control is performed so that the surplus gas is not fed into the boiler 4, and the gas is accumulated in the cargo tank 1. When the steam consumption amount increases, the amount of heavy oil is not increased, but the gas accumulated in the cargo bank 1 is effectively utilized preferentially and positively.