A device for gripping and pulling a length of thermoplastic sheet (S) softened to be moldingly formed at the front and rear edges, which are otherwise inevitably made to be concave, so as to hold said edges substantially straight, the device comprising at least two pairs of lower and upper gripping members (20, 30) arranged to be vertically movable so as to grip the above front and rear edges, and two pairs of pull levers (23, 33) each cooperating with one of the pair of lower and upper gripping members (20, 30) so as to grip the front and rear edges in advance of the gripping by the gripping members and pull to straighten the concaved edges which are then gripped by the gripping members.