A relay lens is used for an imaging system having a main lens. The main lens has a pupil plane. The relay lens, in the order from the subject side to the image side thereof, includes a first lens of negative refractive power, and a second lens of positive refractive power. The first lens has a first principal plane and a second principal plane. The second lens has a third principal plane and a fourth principal plane. The relay lens system satisfies the following formulas: 0.53