To provide an image forming device that integrates control boards while facilitating the assembling and to provide an assembling method for the image forming device.

A scanner 2 is manufactured as a separate unit from an image forming device body 3. Before the scanner 2 is mounted on the image forming device body 3, a read control board 23 is tentatively locked to a tentative lock section provided to the scanner, the quality of the scanner 2 is evaluated in this state, after the quality of the scanner 2 is evaluated, the scanner 2 is mounted on the image forming device body 3, the read control board 23 is removed from the tentative lock section and the read control board 23 is mounted on the forming device body 3.