A fixing device is provided with a fixing device body, a fixing member, and a first attachment section. The fixing device body is adapted to attach to and detach from an apparatus body of a double-sided image forming apparatus, which has a return passage through which a recording medium is returned to an image forming position, and an apparatus body of a single-sided image forming apparatus, which does not have the return passage. The first attachment section is provided at a return passage side in the fixing device body, and is adapted to have a passage surface member, which has a passage surface of an introduction passage through which a fixed recording medium is introduced into the return passage, attached thereto.