An image forming device includes: a rasterizing section, a resolution converting section and a thinning section. The rasterizing section generates, by rasterization, image data having any one of standard resolution, resolution higher than the standard resolution and resolution lower than the standard resolution. The resolution converting section converts the resolution upon rasterization for the generated image data having the higher or lower resolution than the standard resolution into the standard resolution. The thinning section thins the image data having the standard resolution. When thinning the image data of which the resolution is converted higher than upon rasterization in the resolution converting section, the thinning section adjusts thinning strength to become larger upon thinning than when thinning the image data of which the resolution upon rasterization is kept as it is.