In an in-wall heater, in which control equipment 11, 12 is disposed in a side space 10 on one lateral side of a combustion housing 2 within an outer case 1 and an exhaust duct 15 is disposed in a side space 13 on the other lateral side of the combustion housing 2, the cooling performance of each of the side spaces is increased and noise due to a sucked-in sound of room air can be reduced. In a side plate portion of the outer case 1 on both lateral sides, an air intake opening 7, which is vertically long, is provided in a position in a front side portion exposed to a front of a room wall. Each side space 10, 13 within the outer case 1 is formed in a ventilation flue on the suction side where the room air from each of the air intake openings 7 is conducted into the warm air fan 9.