A taking optical system for forming an optical image of an object on the light-receiving surface of an image sensor has at least one optical prism that has an entrance surface, at least three reflective surfaces each curved, and an exit surface, and that receives rays from the object side through the entrance surface, then reflects the rays on the at least three reflective surfaces, and then lets out the rays through the exit surface. The optical prism is formed of a medium having a refractive index of 1.2 or more. An optical aperture is disposed on or near one of the at least three reflective surfaces. At least one of the at least three reflective surfaces is an eccentrically disposed rotation-asymmetric surface. Let the ray traveling from the center of the object through the center of the optical aperture to the center of the image surface be called a base ray, then the intersections of the base ray with the entrance surface, each of the reflective surfaces, and the exit surface all lie approximately on the same plane.