To provide a small-sized imaging optical system with high performance.

The imaging optical system comprises a first lens, a second lens and a third lens in order from an imaging object. The first lens has a meniscus shape directing a convex face to an imaging object side, and has positive refracting power. The second lens directs a convex face at the imaging object side and has positive refracting power. The third lens has a concave shape at an image face side in the vicinity of an optical axis, has an aspheric face being a convex shape to the image face side in an optical axis peripheral part, and has negative refracting power. When a curvature radius of the imaging object side of the second lens is denoted by r4 and a focal length of the whole imaging optical system is denoted by f, relationship of 2.9<r4/f<9 is satisfied.