An image pickup optical system OP includes first and second prisms P1, P2 for bending incident light at an almost right angle. Optical axis AX at the incident surface of first prism P1 and optical axis AX at the outgoing surface of second prism P2 are almost parallel with each other. There is a lens element forming at least one power group on the optical path between first and second prisms P1, P2, wherein a power group closest to the incident surface of second prism P2 is a positive power group. The incident surface of second prism P2 has a concaved surface shape facing the object side, and the following conditional expressions are satisfied: −4.2