The invention provides a rear side shell structure in the form of a back covering that can be attached to a lower structure of a seat without support legs. One representative rear side shell structure and seat unit according to the invention relates to a rear side shell structure for covering a back of a seat, the rear side shell structure including a front surface member and a rear surface member, wherein a reinforcing region in which a space reinforcing member is provided between the front surface member and the rear surface member is provided in a portion of the rear side shell structure, and the front surface member and the rear surface member are directly joined to each other except for in the reinforcement region. Further, the front surface member and the rear surface member are formed of composite members. Also, the space reinforcing member has a honeycomb structure.