A thermal curing device is used to manufacture a high-pressure gas tank by FW method. In the thermal curing device, a tank vessel configured to have a fiber layer formed on its outer surface by winding carbon fibers impregnated with a thermosetting resin is held to be rotatable about a virtual central axis of the tank vessel. In the thermal curing device, the tank vessel is heated by a heating unit during rotation, so as to thermally cure the thermosetting resin in the fiber layer and thereby form a fiber-reinforced resin layer. A bubble removal process for preventing formation of air bubble in the fiber-reinforced resin layer is performed at multiple different stages during such heating.