PURPOSE: To reduce the number of steps in a treating method for the end of a bonded formed product and to improve the external appearance of the product by forming the folded part of a resin core material, simultaneously cutting the trim of the folding margin of the outer periphery of a surface material to be bonded onto a core material with dies.

CONSTITUTION: An upper die 4 is moved down, and core material/surface material 1, 2 are compressed and formed between the projected surface 3a of a lower die 3 and the recess surface 4a of the die 4. At this time, a slide die 6 is moved from the side toward an auxiliary die 5, the excessive ends of the core material/ surface materials 1, 2 are cut and removed by the shoulder part 6a and the edge 5a of the die 5. When the die 6 is further moved, the intermediate part between the folded part a of the material 1 and the outer end b of the material 1 are cut by the projection 3c of the die 3. Subsequently, the end b of the material 1 is isolated from the back surface of the end of the material 2, thereby forming a desired folded part 3c at the end side of the folded part a with the material 2.