there is provided an imaging optical system installed in a mobile communications terminal and a personal digital assistant or utilized in a surveillance camera and a digital camera . the imaging optical system including , sequentially from an object side in front of an image plane : a first lens having positive refractive power and two convex surfaces ; a second lens having negative refractive power and two concave surfaces ; a third lens having positive refractive power and a meniscus shape ; and a fourth lens having a concave object - side surface . the fourth lens has a shape satisfying following condition 1 : 10 & lt ;| r 8 / f |& lt ; 50 condition 1 , where r 8 is a radius of curvature of the object - side surface of the fourth lens , and f is an overall focal length of the imaging optical system .