A stereoscopic optical system includes, at a distal end of an insertion part of an endoscope, a two-path forming optical system for forming two paths of rays involving a parallax, an image forming optical system for forming images out of light travelling along the respective paths of rays in the two-path forming optical system onto a common region, and an image sensor arranged at an image forming position of the image forming optical system. The stereoscopic optical system is further provided with a time-division path switching means that is capable of switching between the two paths of rays in a time-division manner so that only light coming from either one of the two paths of rays formed by the two-path forming optical system enters the image forming optical system.