The room heater has, in an outer case 1, both a first air flow passage that leads to a vent hole 12 through a section in which the main heat exchanger 6 is arranged and a second air flow passage that does not pass through the section in which the main heat exchanger 6 is arranged but leads to the vent hole 12 through a section in which a supplementary heat exchanger 6′ is arranged provided. At least one of the first damper 20 which can freely block the first air flow passage and the second damper 21 which can freely block the second air flow passage is provided, and the first air flow passage or the second air flow passage is blocked in a weak combustion period.