An image forming device includes a main casing and a photosensitive unit having photosensitive bodies arrayed in an array direction. The main casing includes a first side plate having a first primary portion extending in the array direction, and having a first positioning portion and a second positioning portion spaced away therefrom in the array direction. The photosensitive unit includes a second side plate supporting the photosensitive bodies and having a second primary portion extending in the array direction. The second side plate has a third positioning portion subjected to positioning relative to the first positioning portion and has a fourth positioning portion subjected to positioning relative to the second positioning portion. The first and third portions are overlapped with each other and the second and fourth positioning portions are overlapped with each other while the first primary portion and the second primary portion are spaced away from each other.