PURPOSE: To obtain the title device of accurately and readily measuring vapor concentration of steam under low pressure and stably supplying a fixed amount of steam to a steamer by equipping a course of steam piping with a temperature sensor, a vapor pressure control valve and a steam flowmeter, a water piping with a water control valve and a burner with an oil amount control valve.

CONSTITUTION: A course of steam piping between a boiler 1 and a steamer 2 is equipped with a vapor pressure control valve 12 and a steam flowmeter 13, a water piping 19 to the boiler 1 with a water control valve 8, a burner 3 with an oil amount control valve 4 and a steam reservoir 9 to the boiler and a steam piping connected to the reservoir with a temperature sensor 10 for steam to give the objective device having improved supply of a fuel oil to the burner and of water to the boiler.