An image forming device comprising: a main casing including an outer casing and an inner casing inside the outer casing; a plurality of photosensitive bodies each defining a longitudinal direction, and arrayed with a space between neighboring photosensitive bodies in an array direction perpendicular to the longitudinal direction; a photosensitive unit movable relative to the main casing and including the plurality of photosensitive bodies, the photosensitive unit being movable in the array direction between an accommodated position in which the photosensitive unit is accommodated in the main casing and a draw-out position in which the photosensitive unit is moved away from the accommodated position; an exposure unit disposed in the main casing and confronting the photosensitive unit in a confronting direction perpendicular to the longitudinal direction and the array direction, the exposure unit being configured to expose the photosensitive bodies to light; wherein the inner casing comprises a first side plate (34; 34A) supporting the exposure unit and having a first primary portion (37; 37A) extending in the array direction, the first side plate (34; 34A) also having a first positioning portion (38, 49) and a second positioning portion (37; 71) spaced away from the first positioning portion in the array direction; wherein the photosensitive unit comprises a second side plate supporting the photosensitive bodies and having a second primary portion extending in the array direction, the second side plate also having a third positioning portion subjected to positioning relative to the first positioning portion, and a fourth positioning portion (54, 55) subjected to positioning relative to the second positioning portion, characterized in that the first positioning portion and the third positioning portion are overlapped with each other and the second positioning portion and the fourth positioning portion are overlapped with each other in the longitudinal direction while the first primary portion and the second primary portion are spaced away from each other in the confronting direction to provide a space when viewing in the longitudinal direction, wherein the first side plate (34; 34A) further includes a first extension portion extending integrally from the first primary portion (37; 37A) toward the photosensitive bodies and provided with the first positioning portion (38, 49), wherein the second side plate further includes a second extension portion extending integrally from the second primary portion toward the exposure unit and provided with the fourth positioning portion (54, 55) wherein the first primary portion (37, 37A) is provided with the second positioning portion, and wherein the second primary portion is provided with the third positioning portion. a main casing including an outer casing and an inner casing inside the outer casing; a plurality of photosensitive bodies each defining a longitudinal direction, and arrayed with a space between neighboring photosensitive bodies in an array direction perpendicular to the longitudinal direction; a photosensitive unit movable relative to the main casing and including the plurality of photosensitive bodies, the photosensitive unit being movable in the array direction between an accommodated position in which the photosensitive unit is accommodated in the main casing and a draw-out position in which the photosensitive unit is moved away from the accommodated position; an exposure unit disposed in the main casing and confronting the photosensitive unit in a confronting direction perpendicular to the longitudinal direction and the array direction, the exposure unit being configured to expose the photosensitive bodies to light; wherein the inner casing comprises a first side plate (34; 34A) supporting the exposure unit and having a first primary portion (37; 37A) extending in the array direction, the first side plate (34; 34A) also having a first positioning portion (38, 49) and a second positioning portion (37; 71) spaced away from the first positioning portion in the array direction; wherein the photosensitive unit comprises a second side plate supporting the photosensitive bodies and having a second primary portion extending in the array direction, the second side plate also having a third positioning portion subjected to positioning relative to the first positioning portion, and a fourth positioning portion (54, 55) subjected to positioning relative to the second positioning portion, characterized in that the first positioning portion and the third positioning portion are overlapped with each other and the second positioning portion and the fourth positioning portion are overlapped with each other in the longitudinal direction while the first primary portion and the second primary portion are spaced away from each other in the confronting direction to provide a space when viewing in the longitudinal direction, wherein the first side plate (34; 34A) further includes a first extension portion extending integrally from the first primary portion (37; 37A) toward the photosensitive bodies and provided with the first positioning portion (38, 49), wherein the second side plate further includes a second extension portion extending integrally from the second primary portion toward the exposure unit and provided with the fourth positioning portion (54, 55) wherein the first primary portion (37, 37A) is provided with the second positioning portion, and wherein the second primary portion is provided with the third positioning portion.