A strip-shape softened resin sheet (S) which is continuously extruded from a molten resin extruder is cut to a unit resin sheet and a press molded product is manufactured by press-molding the unit resin sheet in a press-molding machine 20. Prior to press-molding the unit resin sheet (U) by the press-molding machine, the unit resin sheet (U) is heated by a heating machine 16. The heating machine 16 comprises a first heating furnace 84 and a second heating furnace 86. The first heating furnace 84 includes a series of heaters 84-3 and 84-4 whose heat source is infrared ray in a far-infrared region and the second heating furnace 86 includes a series of heaters 86-3 and 86-4 whose heat source is the infrared ray in a middle-infrared region. In the first furnace 84, the unit resin sheet (U) is continuously conveyed with a low velocity and is gradually heated by the far-infrared ray up to temperature which is slightly lower than the temperature which is suitable for press-molding the unit resin sheet (U). In the second furnace 86, the unit resin sheet (U) is stopped and is rapidly heated by the middle-infrared ray. By efficiently heating the unit resin sheet (U), a cycle time can be shortened and the production speed can be improved.