In continuous printing of plural sheets of recording paper, a controlling section of an image forming apparatus corrects a writing position for a first sheet of recording paper in accordance with an off-center amount detected by a carrying position sensor with respect to the first sheet of recording paper. A turbofan is arranged upstream from the core nacelle and is coupled to the spool. A density gradient is established between regions adjacent to the surfaces of the housing and the internal regions, thereby reducing the need for reinforcing ribs particularly in a base or mounting portion of the housing. The arm reception portion has a guide portion extending to the retaining portion, and a distal end portion of the lock arm is received between the guide portion and an outer surface of the other article.