Image forming apparatus with arrangement for controlling assembly order

The process frame body has the guide portion, and the development device has the to-be-guided shaft. When the process frame body is unattached to the apparatus main body, the guide portion of the development device is absent, thus preventing proper attachment and positioning of the development device. The convex engagement portion is formed integrally with the development device so as to jut into the attachment path for the process frame body. This prevents attachment of the process frame body when the development device is attached first. If users try to attach the process frame body after attachment of the development device, the convex engagement portion interferes with the process frame body, thus preventing attachment of the process frame body.