An imaging lens includes a first lens having negative refractive power; a second lens having positive refractive power; a third lens having positive refractive power; and a fourth lens having negative refractive power, arranged from an object side to an image plane side. In the first lens, a curvature radius of an image-side surface is positive. In the second lens, curvature radii of the object-side surface and the image-side surface are both negative. In the third lens, a curvature radius of the object-side surface is positive and a curvature radius of the image-side surface is negative. When a whole lens system has a focal length f and the first lens has a focal length f1, the imaging lens satisfies the following conditional expression: −3.0