PURPOSE: To form a thermoplastic sheet surface material without exposing a core material by sufficiently bonding a surface material and the core material without passing them through an bonding process by performing sheet blow molding in such a state that the core material is inserted.

CONSTITUTION: When thermoplastic sheets 1, 1' are held at a predetermined interval by a spacer 2 and held under pressure by clampers 3, 3' to form a closed space S, a core material 8 is preliminarily placed on the sheet l'. Subsequently, the sheets 1, 1' are heated by moving type far infrared heaters 5, 5' and, when the sheets 1, 1' are properly softened, the heaters 5, 5' are moved to be removed and, immediately, a blow molding air nozzle 7 is allowed to pierce through the closed space S to start air blow. Succeedingly, the clamping operation of molds 6, 6' is pertormed. By this constitution, a blow molded product wherein the core material 8 is inserted in the enclosure formed by the surface materials formed to the inner surfaces of molds 6, 6' in a close contact state is obtained.