To provide an optical adapter for taking a stereoscopic video which does not require the drastic readjustment of a focus.

Since a 1st prism 13 is rotated around a rotary shaft S matched with a reflection surface 13m, the focal distance of an optical system 4 is not changed so much and the drastic readjustment of the focus is not required. Since the 1st prism 13 automatically rotates linked with the zoom mechanism 3 of the optical system 4, operability is improved. Furthermore, since the rotary shaft S is positioned in the center of an effective reflection range R on the reflection surface 13m of the 1st prism 13, the focal distance of the optical system 4 is hardly changed particularly.