An image-forming apparatus includes: an optical sensor that includes a light source unit which applies light having a light-emission main wavelength λ to a peripheral face of an image-supporting member, and a light-receiving unit which receives a reflected light thereof, so as to optically detect a toner pattern formed on a peripheral face of the image-supporting member, wherein the image-supporting member has at least one thin-film layer formed on the peripheral face thereof, and the thickness of an outermost surface thin-film layer is set so as to allow a reflectance function R(d) that indicates the relationship between a reflectance R of the peripheral face of the image-supporting member to light having a light-emission main wavelength λ from the light source unit and a thickness d (nm) of the outermost surface thin-film layer of the image-supporting member to satisfy: R(d)≧0.75×{Rmax(d)−Rmin(d)}+Rmin(d).