A transparent film for forming, e.g., a toner image thereon is provided as an optically transparent sheet member having an opaque part. During the image forming, the position and conveyance state of the transparent film are detected optically by utilizing the opaque part. After the image formation, the opaque part is cleared by (i) detaching the opaque part under heating, or (ii) clarifying the opaque part by heating and/or light illumination. Accordingly, the opaque part required for image forming does not adversely affect the final transparent film product, such as an overhead projection film. The clearing of the opaque part may be performed, e.g., along with the fixation of the toner image onto the transparent film.