The tandem photosensitive-member unit includes a plurality of photosensitive-member and a pair of side plates. The plurality of photosensitive-member holding units each extends in a first direction. The plurality of photosensitive-member holding units is arranged in a second direction perpendicular to the first direction. The pair of side plates each extends in the second direction. The pair of side plates is disposed at both ends of the arranged plurality of photosensitive-member holding units with respect to the first direction. The tandem photosensitive-member unit is configured to be detachably mounted as an integral unit in a main body of an image-forming device.