PURPOSE: To provide a compact infrared ray projecting system for range finder with a simple constitution having the reflected light intensity enough for range finding by projecting an infrared ray projected to a half mirror on an object through the half mirror and an objective lens group.

CONSTITUTION: This optical system is provided with a finder optical system, composed of a first lens L1, a second lens L2 and a third lens L3 which compose an objective lens group and a fourth lens L4 composing an eyepiece lens group, and an infrared ray projecting system composed of an infrared ray LED 1, a fifth lens L5 and a half mirror M. Namely, this infrared ray projecting system is composed of the half mirror M arranged on the optical axis of the finder optical system between the third lens L3 and the fourth lens L4, the LED 1 transmitting the infrared ray in the direction of the half mirror M and the fifth lens L5 irradiating the half mirror M with the infrared ray as a convergent light beam. The infrared ray from the LED 1 is reflected by the half mirror M in the direction of the objective lens group and projected on the object by means of the objective lens group.