An air blowing fan supplies an amount of air corresponding to a fan rotation speed to a burner during operation. A check valve has a biasing force that closes an exhaust vent, and is opened and closed in accordance with a relationship between a gas pressure in an exhaust passage and the biasing force. A controller performs a failure diagnosis for detecting an open failure of the check valve when a phenomenon in which the gas pressure is lower than a predetermined normal pressure range is detected in a case where the fan rotation speed is within a predetermined diagnosis rotation speed region. The diagnosis rotation speed region is set within a rotation speed range in which a degree of opening of the check valve changes with an increase in the fan rotation speed when the check valve is normally opened and closed.