PURPOSE: To provide an operating method to solve a problem that binder material is evaporated and a vacuum brazing furnace is stained when the vacuum brazing furnace where the binder material to fix the brazing filler metal is used is operated.

CONSTITUTION: High vacuum is necessary to execute vacuum-brazing to heat resistant steel containing Al, a diffusion pump is used, two series of exhaust piping and changeover valve are arranged, as an operation method, the diffusion pump is bypassed in the evaporation temperature zone 8 of binder in the initial period of the temperature raising. Only both a mechanical booster pump and an oil rotary pump are used, after evaporation, they are changed over to the piping of the diffusion pump to obtain a high vacuum necessary for brazing. Consequently, the diffusion pump can be prevented from being stained by evaporated binder.