An imaging optical system includes: an object-side lens group having, in order from its object side, an object-side positive meniscus lens that is convex on its object side and has positive refracting power, and an object side negative meniscus lens that is convex on its object side and has negative refracting power; and image-side lens group having, in order from its object side, an image-side negative lens, an image-side first positive lens and an image-side second positive lens. The image-side negative lens and the image-side first positive lens adjacent thereto form together a cemented doublet.