1. An imaging lens comprising: a first lens having negative refractive power; a second lens having positive refractive power; a third lens having positive refractive power; a stop; and a fourth lens having positive refractive power, arranged in this order from an object side to an image plane side, wherein said first lens is formed in a shape so that a surface thereof on the image plane side have a positive curvature radius, said second lens is formed in a shape so that a surface thereof on the image plane side have a negative curvature radius, said third lens is formed in a shape so that a surface thereof on the image plane side has a negative curvature radius, said fourth lens is formed in a shape so that a surface thereof on the object side has a positive curvature radius and a surface thereof on the image plane side has a negative curvature radius, and said first lens has a focal length f1 and an Abbe's number νd1 so that the following conditional expressions are satisfied: 
 −75< f 1/ f<− 5.0 
 45< νd 1<70 where f is a focal length of a whole lens system.