To provide an optical system having lesser number of parts and a thin thickness for linearly focalizing light from a point light source and an image-forming device using the same.

This is an optical system having at least a curved reflecting surface 7, and converting the light, which is emitted from almost a point light source 2 eccentrically placed in an across-section crossing the reflecting surface 7 and travels in the cross section, into approximately parallel light, and also almost converging the light from the light source going outside of the cross-section on one line 4 positioned in the cross-section. In such a manner, it is possible to realize an optical system having lesser number of parts and the thin thickness for focalizing the light from the point light source and an image-forming device using the same. Such an image-forming device is, for example, used as an electronic camera of one-piece design or an attachable and detachable printer.