Abstract: A plastic sheet cassette includes a first frame member pivotably coupled to a second frame member, and a plastic sheet is fixedly supported by male and female parts provided on the first and second frame members. A thermoforming apparatus includes a cabinet, a heating source, a vacuum source, a platform, and a fixing frame that supports the cassette so as to be removable, and is vertically movable between the heating source and the vacuum source in the cabinet. The fixing frame can be stopped or released at any desired level in the cabinet and supports the cassette so that the sheet is heated at the upper level by the heating source and is brought into close contact with a prototype at the lower level by the vacuum source for making a mold having the same shape as the prototype.