Vacuum forming apparatus and vacuum forming method

A vacuum forming apparatus has a forming die, a plurality of holding devices, a pressing device and a pair of supplemental holding devices. The forming die has at least one suction hole for creating a vacuum. The holding devices hold a resin sheet that has been softened by heating. The pressing device moves relative to the forming die to close a space therebetween and form an airtight state therebetween. Each of the supplemental holding devices includes a clamp member and a support member. The supplemental holding devices are movably arranged to move toward each other. The clamp members clamp the resin sheet on two sides of a selected sag area where the resin sheet will be made to sag. The support members extend along two side portions of the selected sag area for supporting a bottom surface of the resin sheet where the resin sheet will be made to sag.