To provide the structure of a sintering furnace with which bubbles produced on the bottom face of a muffle are forcedly diffused, and the vibration of the muffle caused by large bursting can be prevented.

A bubble diffusion mechanism in which bubbles produced on the bottom face of the muffle 10 in a cooling part 4 are diffused and elevated before the bubbles are joined to each other and are made large is provided. The bubble diffusion mechanism absorbs cooling water 6 inside a cooling tank 8 with a circulation pump 18, feeds the cooling water to a pipe 16 having holes 16a, and spouts the same from the holes 16a. When the cooling water is fed, and the bubbles are forcedly diffused while they are small, even if the bubbles are burst around the muffle 10, the impact is small, so that the muffle 10 is not greatly vibrated.