An imaging optical system includes, in order from the object side, a first lens group, an aperture stop, and a second lens group. The first lens group includes, in order from the object side, a negative lens of a biconcave shape and a positive meniscus lens with a convex surface facing the object side. The second lens group includes, in order from the object side, a second-front lens group and a second-rear lens group, the second-rear lens group having a positive refractive power as a whole. The second-front lens group includes a cemented lens formed by combing a negative lens with a concave surface facing the object side and a positive lens.