PURPOSE: To omit a water level detection device for a boiler body and make it free of maintenance by calculating the time required for the water level to fall from a set water level in a boiler to a specified water level based on the heat quantity for heating and water temperature detected by a water temperature sensor and controlling supply water means based on the calculated time.

CONSTITUTION: A solenoid valve 3 as supply water means and a supplied water sensor 14 are provided in a supply water channel going to a boiler body 4 and at the same time a water discharge solenoid valve 10 is provided in a water discharge channel which is connected to the boiler body 4. The time T4 which is required for the water level to fall from a supplied water stop position L2 as a set water level in the boiler body 4 to a specified water level given by evaporation, namely minimum water level L3 is calculated based on the heat quantity for heating of a burner 6 and the detected water temperature t1 measured by a supplied water temperature sensor 14, and a control circuit 11 is provided which controls based on the calculated time a pump 12 as the supply water means. And, after a calculated time T4' from the position L to stop supplying water in the boiler body the supply water means 12 is controlled to replenish water in a specified quantity.