An optical system for a color copier in which light images of blue, green and red produced by color separation are formed one on each of three photoconductive elements, then developed by toner of associated colors, and then transferred one upon another onto a paper. A first mirror is disposed in an optical path to which light representative of an image of an original document is led through an image-forming lens and a mirror group. Light reflected by the first mirror is focused onto a first photoconductive element. Light transmitted through the first mirror is split by a second mirror which is located in a position for intercepting a part of the width of a slit for exposure. Light reflected by the second mirror is focused onto a second photoconductive element. Further, light outside the range of the second mirror is reflected by a third mirror to be focused onto a third photoconductive element. The first mirror has a spectral characteristic which reflects light of one of three colors, i.e., blue, green and red while transmitting light of the other two colors. All of the first to third mirrors comprise total reflection mirrors, or only the first mirror comprises a half-mirror. A reflection from the first mirror is of a particular color to which the photoconductive elements show the smallest amount of reaction. Light of wavelengths outside a predetermined range of each color which is to be reflected by any of the mirrors is cut by using the reflection characteristic of the mirror (as well as a transmission characteristic in the case of the half-mirror) and the transmission characteristics of a filter which is associated with the mirror.