PURPOSE: To provide a miniaturized and light weight reading optical system by using a single lens by composing the system of the respectively specified diaphragms and a positive meniscus lens whose concave surface confronts the side of this diaphragm.

CONSTITUTION: A reading optical system including a image forming lens using for a bar code reader is composed of a diaphragm and a positive meniscus lens whose concave surface confronts the side of the diaphragm. The order of the arrangement is e.g. a meniscus lens and the diaphragm in order from an object side. In this case, by defining the sum of the interval between the diaphragm and the lens and the lens thickness is d1+d2, the focal length is (f) and the radius of curvature of the meniscus lens on the object side is r1, the conditions: 0.15<(d1+d2)/f<0.41, 0.35<(d1+d2)r1<0.85 are satisfied. Consequently, the reading optical system having a necessary and sufficient performance as a bar code reader is obtained, the device is miniaturized, light-weight and the cost is reduced.