PURPOSE: To provide the optical system which has a variable power ratio as large as ≥2 times, is compact and has high performance at all magnifications by increasing the focal length on a high magnification side at the time of changing the spacing in a part within the lens system.

CONSTITUTION: This optical system is made of the intermediate constitution of a zoom lens and a fixed focal length lens system. The optical system has a reading lens and a movable mirror for changing a distance between the object and the image between the fixed object and image plane. The reading lens is constituted by arranging, successively from the object side, a front group having a negative focal length and a rear group having a positive focal length. The focal length is increased by decreasing the distance between the front and rear groups and the power is varied by decreasing the distance between the object and the image at the time of varying the low magnification to the high magnification; in addition, the conditions of equation are satisfied. In the equation, fmin is the min. value of the focal length of the entire system on the low magnification side; fmax is the max. value of the focal length of the entire system on the high magnification side; K is the variable power ratio of the magnification to be used; fI is the focal length of the front group.