A heat source apparatus comprising: a burner; a combustion box for enclosing a combustion space for air-fuel mixture ejected from a combustion surface of the burner; a heat exchanger, disposed in the combustion box, for heating cold water; a water jacket disposed in that portion of the combustion box which lies between the burner and the heat exchanger, the water jacket being connected to the heat exchanger in order to cool the combustion box; an electrode component mounted in that portion of a one-side side plate of the combustion box which lies between the burner and the heat exchanger, the electrode component comprising: an electrode (61, 62, 63) which protrudes into the combustion box through a through-hole (21a, 21b) formed in the one-side side plate; and an insulator (64 1, 64 2) through which the electrode is fixedly inserted, the insulator having a flange portion which overlaps with a portion of outside surface around the through-hole in the one-side side plate and, in a state in which a packing is interposed between the flange portion and the one-side side plate, a clamp which overlaps with an outside surface of the flange portion is fastened to the one-side side plate, whereby the electrode component is mounted in position on the one-side side plate; upper and lower water tubes constituting a part of the water jacket and disposed on the one-side side plate on upper and lower sides, respectively, of the position of mounting the electrode component; characterized in: that, in case the upper and lower water tubes are disposed on an inside surface of the one-side side plate, upper and lower side edge portions of the packing are in contact with those upper and lower swelled-out portions formed in the one-side side plate which are swollen outward for respectively receiving, into the swelled-out portions, the upper and lower water tubes; and that, in case the upper and lower water tubes are disposed on an outside surface of the one-side side plate, the upper and lower side edge portions of the packing are in contact with the upper and lower water tubes. a burner; a combustion box for enclosing a combustion space for air-fuel mixture ejected from a combustion surface of the burner; a heat exchanger, disposed in the combustion box, for heating cold water; a water jacket disposed in that portion of the combustion box which lies between the burner and the heat exchanger, the water jacket being connected to the heat exchanger in order to cool the combustion box; an electrode component mounted in that portion of a one-side side plate of the combustion box which lies between the burner and the heat exchanger, the electrode component comprising: an electrode (61, 62, 63) which protrudes into the combustion box through a through-hole (21a, 21b) formed in the one-side side plate; and an insulator (64 1, 64 2) through which the electrode is fixedly inserted, the insulator having a flange portion which overlaps with a portion of outside surface around the through-hole in the one-side side plate and, in a state in which a packing is interposed between the flange portion and the one-side side plate, a clamp which overlaps with an outside surface of the flange portion is fastened to the one-side side plate, whereby the electrode component is mounted in position on the one-side side plate; upper and lower water tubes constituting a part of the water jacket and disposed on the one-side side plate on upper and lower sides, respectively, of the position of mounting the electrode component; that, in case the upper and lower water tubes are disposed on an inside surface of the one-side side plate, upper and lower side edge portions of the packing are in contact with those upper and lower swelled-out portions formed in the one-side side plate which are swollen outward for respectively receiving, into the swelled-out portions, the upper and lower water tubes; and that, in case the upper and lower water tubes are disposed on an outside surface of the one-side side plate, the upper and lower side edge portions of the packing are in contact with the upper and lower water tubes.