To accurately form a flange even by using resin whose glass-transition temperature is high, to uniformize formed products and to reduce a manufacturing cost.

A sheet 8 is formed by cooling the melted resin extruded from a flat die 11A by using an elastic roll 12 having an elastic body. Then, a recessed part 8A is thermally formed by bringing the heated sheet 8 into contact with the outside circumferential surface of a molding drum 32 having cavities 31 at the outside circumferential surface part thereof and sucking it from the cavities 31. The recessed part 8A is cut off from the sheet 8 to manufacture the flange 2.