An imaging optical system 100 has an imaging side prism 102 for bending incident light at about 90 degrees for reflection, and an image sensor 105 having a light receiving surface opposing to an exit surface 102b of the imaging side prism 102. At least one of an incident surface 101a of an incident side prism 101 and an incident surface 101a of the imaging side prism 102, or at least one of an exit surface 101b of the incident side prism 101 and the exit surface 102b of the imaging side prism 102 has an optical power. An arrangement relation between the exit surface 102b of the imaging side prism 102 and the image sensor 105 is established to satisfy the conditional formula (1): 0.0≦d/a