Imaging lens

1. An imaging lens comprising:

a first lens having positive refractive power;

a second lens having positive refractive power;

a third lens having negative refractive power;

a fourth lens having negative refractive power;

a fifth lens;

a sixth lens;

a seventh lens;

an eighth lens; and

a ninth lens having negative refractive power, arranged in this order from an object side to an image plane side,

wherein said eighth lens and said ninth lens are arranged at an air interval,

said ninth lens is formed in a shape so that a surface thereof on the image plane side has an aspherical shape having an inflection point, and

the following conditional expression is satisfied:

0.2< R 9 r/f< 1.0,

where f is a focal length of a whole lens system, and R9r is a paraxial curvature radius of an image plane side surface of the ninth lens.

2. The imaging lens according to claim 1 , wherein said second lens has a focal length f2 so that the following conditional expression is satisfied:

0.3< f 2/ f< 1.2,

where f is a focal length of a whole lens system.

3. The imaging lens according to claim 1 , wherein said third lens has a focal length f3 so that the following conditional expression is satisfied:

−3.5< f 3/ f<− 1.0,

where f is a focal length of a whole lens system.

4. The imaging lens according to claim 1 , wherein said third lens has a focal length f3 and said fourth lens has a focal length f4 so that the following conditional expression is satisfied:

4< f 4/ f 3<20.

5. The imaging lens according to claim 1 , wherein said ninth lens has a focal length f9 so that the following conditional expression is satisfied:

−4.0< f 9/ f<− 0.5,

where f is a focal length of a whole lens system.