A thermoforming device using a hot plate heating comprising: a frame which has a base portion, a peripheral wall portion erected on a periphery of the base portion, and a concave portion formed by the base portion and the peripheral wall portion; a hot plate which has a heating surface facing the concave portion of the frame and heater that heats the heating surface, the heating surface being formed in a flat plate shape having a smooth flat surface, the hot plate having an upper surface on an opposite side of the heating surface of the hot plate, the heater being provided on the upper surface, the hot plate forming a shape greater than that of the frame, the hot plate being provided with a plurality of ventilation holes communicating with the heating surface at the frame side; a decompression unit which is connected to the frame, fixes a sheet to an upper end of the peripheral wall portion of the frame, and decompresses an inner portion of the concave portion covered by the sheet; a decompression unit which is connected to the hot plate and sucks the heating surface side; a unit which is connected to the hot plate and opens the heating surface side to an atmosphere or pressures the heating surface side; an adsorption and heating control unit which performs an adsorption and heating operation of the sheet by the hot plate in a state in which the frame and the hot plate come into close contact with each other with the sheet interposed therebetween; a decompression control unit which performs a decompression operation by the decompression unit in the concave portion covered by the sheet; and a molding operation control unit which concurrently performs the adsorption and heating operation by the adsorption and heating control unit and the decompression operation by the decompression control unit, stops the adsorption and heating operation of the sheet by the hot plate after a predetermined time from the start of the operation, and opens a portion between the hot plate and the sheet to the atmosphere or pressures the portion. a frame which has a base portion, a peripheral wall portion erected on a periphery of the base portion, and a concave portion formed by the base portion and the peripheral wall portion; a hot plate which has a heating surface facing the concave portion of the frame and heater that heats the heating surface, the heating surface being formed in a flat plate shape having a smooth flat surface, the hot plate having an upper surface on an opposite side of the heating surface of the hot plate, the heater being provided on the upper surface, the hot plate forming a shape greater than that of the frame, the hot plate being provided with a plurality of ventilation holes communicating with the heating surface at the frame side; a decompression unit which is connected to the frame, fixes a sheet to an upper end of the peripheral wall portion of the frame, and decompresses an inner portion of the concave portion covered by the sheet; a decompression unit which is connected to the hot plate and sucks the heating surface side; a unit which is connected to the hot plate and opens the heating surface side to an atmosphere or pressures the heating surface side; an adsorption and heating control unit which performs an adsorption and heating operation of the sheet by the hot plate in a state in which the frame and the hot plate come into close contact with each other with the sheet interposed therebetween; a decompression control unit which performs a decompression operation by the decompression unit in the concave portion covered by the sheet; and a molding operation control unit which concurrently performs the adsorption and heating operation by the adsorption and heating control unit and the decompression operation by the decompression control unit, stops the adsorption and heating operation of the sheet by the hot plate after a predetermined time from the start of the operation, and opens a portion between the hot plate and the sheet to the atmosphere or pressures the portion.