PURPOSE: To downsize a lens system including the outer diameter, and enhance the optical performance up to a screen peripheral part by setting the lens first face and lens final face into concave faces, and satisfying a specified condition.

CONSTITUTION: The lens system is formed out of a first lens group having positive refracting power, a second lens group having negative refracting power, a diaphragm, a third lens group having positive refracting power, and a fourth lens group having positive refracting power in the order from an object side, the lens first face and the lens final face are set to concave faces, and the following condition is satisfied: 0.25<fR/fF<1.3, wherein fF is the composed focus distance of the lens groups in the front of the diaphragm, and fR is the composed focus distance of the lens groups in the rear of the diaphragm. When the lens first face is concave rather than convex, the peripheral light quantity is relatively easily ensured. Therefore, when the first face is set to be the concave face, a vicious cycle such that the lens diameter is increased only to ensure the light quantity, can be avoided, the distance from the lens first face to the incident pupil position can be shortened, and the lens system can be reduced in size.