To improve the efficiency of a heating work by rapidly heating up a cast product, in a heat treatment method of the cast product as the pre- treatment of the cast product.

The heated air is made flow into a fluidized bed furnace 10 to heat the fluidized bed 2. In this state, the cast product 3 is taken out by opening molds and hung down with a hanging rope 51. Further, the cast product is buried in the fluidized bed at 540-550° for about 1 hr to execute a solution treatment of the cast product. Thereafter, the cast product is pulled up from the fluidized bed and water-cooled. Again, the cast product is hung down and buried into the fluidized bed at about 150° for about 1 hr to execute an aging treatment of the cast product. Then, since the surface of the cast product is ground by the fluidized medium in the fluidized bed at the same time with the heating, a shot process as the after-process is omitted and this cast product can be worked and coated.