To improve a mixing property of a mixed gas in a premixing chamber 14.

An extension cylinder 100 and an annular plate 101 are disposed in the premixing chamber 14. The cylinder 100 which extends a primary air supply path 15 to the upper side is provided for the purpose of elongating the distance from an air supply port B to a catalytic combusting portion 6. The plate 101 is disposed at the downstream side of an evaporated fuel supply port A and the port B to throttle the mixed gas supplied to the combusting portion 6 by a gap C formed between itself and the cylinder 100. The inner pressure is raised at the upstream of the plate 101 where evaporated fuel and primary air are supplied to throttle the mixed gas when passing through the gap C, thereby uniformly diffusing the mixed gas at the downstream where pressure is low. Thus, the mixing property of the mixed gas is improved.