A combustion chamber inside a combustion box has disposed therein a burner positioning frame having: a rear frame part positioned on a rear side of a burner-disposed portion; and a burner positioning frame having side frame part positioned on each laterally outside of the burner-disposed portion. The rear frame part has formed therein a laterally elongated drawn part which is dented rearward. The secondary air flowing from the distribution holes positioned in such a portion of the partition plate as is closer to the rear end of the partition plate is restrained by the drawn part from flowing upward from between the partition plate and a lower edge of the rear frame part through a clearance between the rear frame part and a rear plate part of the combustion box.