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, being connected to the heat exchanger in order to cool the combustion box; - an electrode component mounted in a portion of a one-side side plate of the combustion box, 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 (61, 62, 63) is fixedly inserted, wherein - water tubes constituting a part of the water jacket are disposed on the one-side side plate in the neighborhood of position of mounting the electrode component; characterized in that: - the water jacket is disposed in that portion of the combustion box which lies between the burner and the heat exchanger; - the 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 insulator having a flange portion which overlaps with a portion of outside surface around the through-hole (21a, 21b) 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 wherein, in case the water tubes are disposed on an inside surface of the one-side side plate, a part of the clamp is in contact with that disposed portion of the water tubes which is on the one-side side plate; and wherein, in case the water tubes are disposed on an outside surface of the one-side side plate, a part of the clamp is in contact with the 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, being connected to the heat exchanger in order to cool the combustion box; - an electrode component mounted in a portion of a one-side side plate of the combustion box, 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 (61, 62, 63) is fixedly inserted, wherein -- 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 (61, 62, 63) is fixedly inserted, wherein - water tubes constituting a part of the water jacket are disposed on the one-side side plate in the neighborhood of position of mounting the electrode component; - the water jacket is disposed in that portion of the combustion box which lies between the burner and the heat exchanger; - the 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 insulator having a flange portion which overlaps with a portion of outside surface around the through-hole (21a, 21b) 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