PURPOSE: To provide a collimation optical system wherein an observer can observe in any direction, and in the same direction even when a lens barrel is moved.

CONSTITUTION: Objective optical systems 1a, 1b which are turned so as to be capable of being turned around a prescribed axis Ax3 from the primary image of an object on a focusing glass 2. Then, the image of light from the primary image is formed again by relay optical systems 5a, 5b, 5c. An observer observes the image via an eyepiece optical system 7. At this time, the relay optical systems are provided with first deflection means 3a to 3c which deflect the light from the primary image along a prescribed axis and with a second deflection means 6 which is installed on the prescribed axis so as to be capable of being turned and which deflects light from the first deflection means.