A forced flue type combustion device in which an orifice is provided in at least one of an air supply passage and an exhaust passage, and in which the existence/nonexistence of a clog is determined on the basis of a detected pressure difference between the pressures on the upstream and downstream sides of the orifice, is arranged so that the existence/nonexistence of a clog can be determined with accuracy even during weak combustion with a low rotational speed of the combustion fan. Processing for increasing the rotational speed of the combustion fan is intermittently executed during weak combustion. During weak combustion, clogging determination processing based on the detected pressure difference value is executed only when the rotational speed of the combustion fan is increased. Clogging determination processing is started after the rotational speed of the combustion fan has become stable.
