Abstract: A distortion-free and compact image-forming optical system with a minimal number of optical components which provides a wide image area and which is favorably corrected for ray aberrations and arranged in a compact structure by folding an optical path. A pentagonal prism member (5) has a first surface (1), a second surface (2), a third surface (3), and a fourth surface (4). The first surface (1) and the fourth surface (4) are refracting surfaces. The second surface (2) and the third surface (3) are reflecting surfaces. The first surface (1) is formed from a spherical surface. The second surface (2), the third surface (3) and the fourth surface (4) are formed from three-dimensional surfaces. Light rays from an object plane O provided on the back side of a cover glass (6) pass through the cover glass (6) and enter the prism member (5) through the first surface (1). The incident light rays are reflected successively by the second surface (2) and the third surface (3).