Thermoplastic plate bending method, working jig, and concave thermoplastic plate

A jig used at the time of putting a thermosoftening plate in a heated state and subjecting the thermosoftening plate to bending under a self-weight, the jig comprising:

• a frame body which is in contact with at least part of a peripheral edge of the thermosoftening plate and holds the thermosoftening plate;
• and
• tension generation means for pulling the peripheral edge of the thermosoftening plate and applying tension in a planar direction, wherein
• the tension generation means includes
• a plurality of weights which are arranged around the frame body,
• linear members which connect the weights and the peripheral edge of the thermosoftening plate, and
• a direction change member which is attached to the frame body and changes force in a vertical direction due to the weights, for which the linear members are arranged, into tensile force in the planar direction for the thermosoftening plate, and
• a furnace, wherein when the frame body is lowered into the furnace with a predetermined temperature, the thermosoftening plate bends with tensile force in the planar direction for the thermosoftening plate applied, and when the thermosoftening plate reaches a predetermined bent state having a predetermined radius, the lowering of the frame body and the heating of the furnace are stopped.