The related container forming device comprises: a forming die with an accommodation part for the container material, a holding part to compress the thermoplastic sheet and hold them near the accommodation part, a forming device to stack and stick on the material, a modifying device to cut the thermoplastic sheet and form the container, and a negative-pressure release device to release the negativepressure between the thermoplastic sheet and the holding part by pressure less than normal atmospheric pressure. This invention can improve production speed and product quality even for much thin sheet.