To reduce aberrations, especially curvature of the field to each light beam from each light source to a very small size and the reduce influence of the heat from the light source to a low level, when the multi-beam scanning is performed, by setting a collimator lens to which a prescribed conditional expression is satisfied by disposing a positive lens, a negative and positive cemented lens, the positive lens and the positive lens in the order from a parallel luminous flux side.

While disposing the first positive lens L1, a second lens L2 and a third lens L3, the properties of which are negative in one side, positive in the other side and negative by combining of them, a positive fourth lens L4 and the positive fifth lens L5 turning a concave surface to a light source side in the order from the parallel luminous flux side, the second lens L2 and the third lens L3 are joined, and the conditional expression Bf/f>0.8 (Bf: back focus of the lens whole system, f: focal length of the lens whole system) is satisfied.