1. An image forming apparatus comprising: a smoothing process part that, based on a density of a peripheral pixel adjoining a target pixel on edge, when a density change between both sides of the target pixel in a sub scanning direction is larger than a density change between both sides of the target pixel in a main scanning direction, mutually changes a density of a plurality of pixels after conversion of pseudo high resolution in the sub scanning direction corresponding to the target pixel, and when a density change between both sides of the target pixel in the sub scanning direction is not larger than a density change between both sides of the target pixel in the main scanning direction, sets same density for the density of the plurality of pixels after conversion of pseudo high resolution in the sub scanning direction corresponding to the target pixel; and an exposure controlling part that performs, based on the density of the plurality of pixels after conversion of pseudo high resolution obtained by the smoothing process part, exposure controls about the plurality of pixels after conversion of pseudo high resolution, wherein: the smoothing process part, based on the density of the peripheral pixel adjoining the target pixel on edge, even in case that the density change between both sides of the target pixel in the sub scanning direction is larger than the density change between both sides of the target pixel in the main scanning direction, when a difference of the density change between both sides of the target pixel in the sub scanning direction and the density change between both sides of the target pixel in the main scanning direction is smaller than a specified value, sets the same density for the density of the plurality of pixels after conversion of pseudo high resolution in the sub scanning direction corresponding to the target pixel.