PURPOSE: To obtain proper boiling temperature regardless of temperature distribution in the bathtub by calculating heat quantity required for boiling from the quantity of water in the bathtub and water temperature and determining the combustion time of a burner.

CONSTITUTION: When a water level is detected by means of a hydraulic detector P and is compared with a water level h set from a water-level setting section and agrees with the water level h, a water valve W is closed. Water temperature T at that time is detected by means of a water-temperature detector T, the size (a) of the bathtub corresponding to the horizontal area of the bathtub is set by means of the setting section (a) of the size of the bathtub, boiling temperature θ is set by means of a boiling-temperature setting section θ, the heat quantity Q required for boiling water in the bathtub up to the boiling temperature θ is obtained according to the formula Q=P×(a)×(θ-T), the combustion time (t) of the burner is acquired according to the formula t=Q/q because the capacity (q) of the burner is set to a burner capacity setting section, and a gas valve G is closed by means of a timing section t' after time (t) elapses.