Provided is an inexpensive image forming apparatus that reduces wasteful operation and is capable of determining whether a fixing unit is attached properly. The image forming apparatus includes an image forming apparatus main body, and a fixing roller that fixes a pattern composed of toner on paper by heating when conveying the paper on which the pattern is formed, and uses a fixing unit that can be attached to and detached from the image forming apparatus main body. A temperature sensor for detecting the temperature of the fixing roller is provided in the fixing unit. The image forming apparatus includes a control unit that, after the fixing unit is attached, determines whether or not the fixing unit is attached properly to the image forming apparatus main body according to a magnitude relationship between the temperature detected by the temperature sensor and a preset first threshold value.