A tandem color laser printer, in which processing units can be smoothly attached to and detached from its casing, is provided while an adequate accuracy of exposure to photosensitive drums is maintained. In the color laser printer, scanning units are fixed to the printer casing. The processing units and the scanning units are alternately aligned and overlapped. A top cover is provided to a side opposite to a transfer position where the photosensitive drums contact a conveyor belt so as to freely open and close with respect to the printer casing. With this structure, the adequate accuracy of exposure by the scanning units can be maintained. The processing units can be attached to and detached from the printer casing via an upper opening of the printer casing defined by the opened top cover. Thus, the processing units can be smoothly attached to and detached from the printer casing without being interfered by the scanning units.