To provide an ocular optical system made compact when a telescope and binoculars are not used in spite of its employing a Porro type image inversion optical system.

A pair of ocular optical systems 1a and 1b is provided with the image inversion optical systems (Porro I type) 2a and 2b constituted so that the cross line of the respective extension surfaces of a 1st reflection surface 3 and 2nd reflection surfaces 4a and 4b forming a right angle with the reflection surface 3 and the cross line of the respective extension surfaces of 3rd reflection surfaces 5a and 5b and 4th reflection surfaces 6a and 6b forming a right angle with the reflection surfaces 5a and 5b are arranged at a position where they are distorted and the reflection surfaces 4a and 4b become adjacent within an identical plane surface. Then, a real image formed by objective optical systems 7a and 7b arranged between the 1st reflection surface 3 and the 2nd reflection surfaces 4a and 4b is enlarged and observed through eyepiece optical systems 8a and 8b arranged behind the 4th reflection surfaces 6a and 6b. The 1st reflection surface 3 is formed as one reflection mirror being common to both optical systems 1a and 1b and held so that it can be tilted with respect to the 2nd reflection surfaces 4a and 4b.