To provide an image formation device that enables high definition printing while suppressing deterioration of a wavefront aberration generated when stopping down a spot diameter to 40 μm or less.

A scanning optical system is composed of at least two or more elements of a lens, and is composed of a first group with a power in a main scanning direction and a sub scanning direction and a second group with a power in the sub scanning direction. The scanning optical system is configured to calculate a condition where in any image height, Δφ_n denotes a change amount in a light beam width in the main scanning direction with respect to a power φ_n in the sub scanning direction when a principal ray passes an n-th group, ΔL_n denotes a change amount in the light beam width in the main scanning direction with respect to a distance L_n from a sub scanning front side principal point position to a reflection point of deflection means when the principal ray passes the n-th group, and h_n denotes a light beam half-width value in the sub scanning direction when the principal ray passes the n-th group.