PURPOSE: To prevent screaching noise caused by repeated expansion and contraction of burners, by absorbing thermal expansion in combustion casings caused by conducted heat from the outer edges on the upper surfaces of burners and radiation heat from flames, by moving it to the thrust direction on the upper surfaces of burners.

CONSTITUTION: A big burner 9 and a small burner 10 are composed of molded bodies C into split tubes having nearly U-shaped sections, and frame bodies E which are of wire net covered on the combustion surfaces on the upper surfaces of burners, and the outer edges of frame bodies E are downwardly folded, unitarily caulked to the open edges of bodies C at the positions E1. When one or both burner 9 and/or 10 is/are burned or stopped during combustion for the purpose of controlling the temperature, thermal expansion and contraction are repeated in combustion casings 11 by heat conducted from outer edges A and B on the upper surfaces of each burner 9 and burner 10, and radiation heat from flames. The combustion casings 11 are fitted to the straight-line parts E in the longitudinal direction of each burner 9 and 10 only by the folded parts 11b. Accordingly, the upper surfaces of each burner 9 and 10 move to the longitudinal direction only, so that screaching noise caused by sliding of burr can be prevented.