An image forming apparatus adjusts a gamma-LUT of a gamma correction circuit in accordance with density data on a measurement image formed on a photosensitive drum. A CPU selects a conversion table in association with the image formation condition such as laser power of a semiconductor laser, fixing temperature of a fixing device, or a charge in a developer. A luminance/density converting portion converts luminance data on the measurement image into density data using the conversion table selected by the CPU. The CPU adjusts a contrast potential and a gamma-LUT using this density data.