To provide a multibeam scanner in which the main scanning effective range is secured as broad as possible, the composite deviation of a group of a plurality of laser beams is reduced, the composite deviations produced or increased in time is eliminated, and to provide an image forming apparatus.

The multibeam scanner includes a scanning means 104 which scans a group of m×n beams, which is made by combining laser beams generated by a plurality n of laser arrays 101, 102; a deviation amount detection means 107, which detects from a beam detection signal the composite deviation quantity dT5 in the main scanning direction x between laser arrays, the composite deviation quantity dT6 in a subscanning direction y, and the overall deviation quantity dT2 of the beam position in the y-direction after composition; a memory 108, which holds initial adjustment value (dot delay table) of the write starting point in the x-direction of each beam; a main scanning correction means 109, which corrects the composition deviation in the x-direction on the basis of the initial adjustment value and the composition deviation quantity dT5 in the x-direction; sub scanning correction means 110A, B, 113 which correct the composition deviation in the y-direction, on the basis of the composition deviation quantity dT6 in the y-direction; and beam incident angle correction means 111, 113 which correct the overall deviation in the y-direction of the combined beam group, on the basis of the overall deviation quantity dT2.