A dope, prepared from cellulose triacetate (TAC), solvent and the like, is cast from a casting die onto a belt to form a casting film, which, thereafter, is peeled as a wet film (74) from the belt. The wet film (74) is transported to a tenter device, in which both side edge portions are clipped by tenter clips (100) and the firm is dried with stretching in the widthwise direction. The tenter clips (100) (temperature, 0 to 60 DEG C) has a support surface (100a) for supporting the side edge portion of the wet film (74) thereon. A surface tension of the support surface (100a) is in the range of3.0 x 10^-2 to 3. 3 x 10^-2 N/m, a surface roughness Ra 0.05 to 1.0 m, and a surface hardness 400 to 800 Hv. Thus, the present invention results in reducing the adhesion of the foreign materials on the surface.