A field of view direction changing optical system comprising an incident face intersecting at right angles with the optical axis of the light from the observation field of view forming an angle .theta. with respect to an optical axis of an image transmitting optical system, a first reflecting face forming an angle .theta./2 with respect to said incident face and a prism having a second reflecting face parallel to said incident face or coincident with it, in order to prevent the occurrence of astigmatism and to enable that the positioning adjustment at assemblage can be done easily. On the second reflecting face a parallel plane glass is secured and on said parallel plane glass a plano-concave lens aligned to said optical axis forming an angle .theta. with respect to the optical axis of the image transmitting optical axis may be secured.