A thermoforming apparatus includes a female die and a movable plug for thermoforming a softened thermoplastic resin sheet. The female die includes a rim forming recess in the shape of a bowl-shaped recess, communicating with a bottom opening and forming a bottom wall of the bowl, an outer wall, a bottom wall and an inner wall of a rim of the bottom, and a bottom plate movable forward and backward relative to the rim forming recess. The softened thermoplastic resin sheet is thrust by the plug while the bottom plate is held at a first stop position. The thermoplastic resin sheet is brought into contact with an inner circumferential surface of the bowl-shaped recess and then the bottom plate is moved to a second stop position so that a rim is formed.