PURPOSE: To make a focusing mechanism compact by positioning the back focus of a collimator lens on a more image forming lens side than the last surface thereof on the emitting side of luminous flux and positioning the front focus of an image forming lens in the image forming lens.

CONSTITUTION: This system is constituted of the collimator lens 11 and the image forming lens 12 and a diaphragm S is positioned in the image forming lens 12 group. Then, the lens 11 is constituted of five groups and five lenses and the lens 12 is constituted of eight groups and eight lenses. In such a case, a telecentric system is constituted by using an inverted telescope type that the front focus is made to enter the lens as the image forming lens 12 and using a type that the back focus is positioned outside the lens as the lens 11 so as to avoid the change of magnification at a using time. Therefore, even when a focusing action is executed by moving only the lens 12, the magnification is not changed. Besides, since the aberration of the lens 12 is also corrected so that the most excellent performance is obtained when the parallel luminous flux is made incident on in the case that an object distance is infinity, the focusing action can be executed by moving only the lens 12.