PURPOSE: To make a sheet skin to be sucked and closely contacted with a bottom force surely by providing a groove-molding plate projected from the molding face of the bottom force and an elevator capable of raising and lowering said groove-molding plate in synchronization with the sucking operation of the sheet skin.

CONSTITUTION: In a sheet skin 1 having been set when a groove-molding plate 12 is in the position of an upper end, the groove-molding plate 12 is lowered to the position of lower end, while the sucking chamber 2b in the bottom force 2 is vacuum-sucked. The sheet is rendered baggy by said lowering, and further the sheet skin 1 at the baggy part is sucked by vacuum suction, and then said skin is fixed to a bottom force 2 in the state where it is positioned along the groove-molding plate 12.