PURPOSE: To improve thermal efficiency by continuously providing suspending flaps on the lower ends of fins provided on the outer periphery of a water passing pipe in a sub heat exchanging part in a gas combustor formed by com bining a gas burner with a two-stage heat exchanger.

CONSTITUTION: A main heat exchanging part 1a is constituted by providing fins 12a... on the outer periphery of a water passing pipe 11, and also a sub heat exchanging part 1b is constituted by providing fins 12b... on the outer periphery upstream from the main heat exchanging part 1a in the water passing pipe 11. And the main heat exchanging part 1a and the sub heat exchanging part 1b are subsequently arranged on the downstream side of a gas burner 3 in an exhaust passage in this order, and combustion exhaust gas is cooled by the sub heat exchanging part 1b to the dew point or less after absorbing sensible heat of combustion exhaust gas by the main heat exchanging part 1a, and the latent heat is absorbed from the combustion exhaust gas. In this case, suspending flaps 5 are continuously provided on the lower ends of the fins 12b in the sub heat exchanging part 1b. Thereby drain stuck on the front surface of the sub heat exchanging part 1b is quickly moved to the suspending flaps 5, so as to prevent thermal efficiency from being deteriorated.