To prevent the deformation of a muffle in the width direction and in the vertical direction.

Both ends of support rollers 11 are arranged by being supported on bearings 12 provided on a counter 1 in a manner to be freely peripherally rotatable in the circulation direction of an endless belt 5 at predetermined intervals. The muffle 2 is supported on the rollers 11 longitudinally in agreement with the direction of the arrangement of the rollers 11. When the muffle 2 is thermally expanded/contracted, the support rollers 11 are rotated following the expansion/contraction. Accordingly resistance (frictional resistance) to the expansion/contraction movement and difference in local resistance become substantially zero, and the muffle 2 expands/contracts straightly in the longitudinal direction without deforming in the width direction or vertical direction.