An imaging optical system including, in order from an object side to an image side, a first lens group having a negative refractive power, an aperture stop, a second lens group having a positive refractive power, is provided. The first lens group includes in order from the object side a first lens having a positive refractive power and a meniscus shape turning a convex surface to the object side, a second lens having a negative refractive power, and a third lens having a positive refractive power. A lens surface positioned closest to the image side in the first lens group is a concave surface to the image side, and a lens surface positioned closest to the object side in the second lens group is a concave surface to the object side.