An imaging lens includes a first lens; a second lens; and a third lens. A curvature radius of an object-side surface and an image plane-side surface of the first lens is positive. The second lens is formed such that a curvature radius of an object-side surface is negative and a curvature radius of an image plane-side surface is positive. A curvature radius of an object-side surface and an image plane-side surface of the third lens is positive. When the first lens has a focal length f1, the second lens has a focal length f2, the third lens has a focal length f3, a composite focal length of the first lens and the second lens is f12, and a composite focal length of the second lens and the third lens is f23, the imaging lens satisfies the following expressions: f1