An image-taking optical system for forming an optical image of an object on the light-receiving surface of an image sensor includes a plurality of optical blocks. The optical blocks each have a prism unit provided with a curved surface and an optical unit composed of a member different from a member composing the prism unit. The prism unit uses the curved surface as a total reflection surface and also as a transmission surface. The optical unit has near the curved surface a curved surface identically or substantially identically shaped therewith.