An imaging lens includes a first lens having positive refractive power; a second lens having positive refractive power; and a third lens having positive refractive power, arranged in the order from an object side to an image plane side. The first lens and the third lens have object-side surfaces and image plane-side surfaces, curvature radii of which are both positive. When the first lens has refractive power P1, the second lens has refractive power P2, the third lens has refractive power P3, the first lens has a focal length f1, and the third lens has a focal length f3, the imaging lens satisfies the following conditional expressions: 5.0