In a heat source apparatus in which an electric component having an electrode and an insulator is mounted on a side plate of a combustion box, in a state in which a packing is interposed between a flange portion of the insulator and the side plate, by fastening a clamp which overlaps with an outer surface of the flange portion, to the side plate, the temperature rise in the clamp is restrained.Upper and lower water tubes constituting a part of a water jacket are disposed on the side plate on upper and lower sides of the position of mounting the electrode component. In case the upper and lower water tubes are disposed on an inside surface of the side plate, the upper and lower parts of the clamp are brought into contact with those disposed positions of the water tubes which are in upper and lower portions of the side plate and, in case the upper and lower water tubes are disposed on an outside of the side plate, the upper and lower parts of the clamp are brought into contact with the upper and lower water tubes.