An object of the present invention is to provide a seat unit structure, a seat unit, and an attachment method thereof in which component reinstallation does not arise when the seat unit is attached in an aircraft. One representative seat unit structure according to the present invention relates to a seat unit structure configured to support a seat, wherein the structure has a lower structure having a monocoque structure and defines an outermost shell in at least one direction of the seat unit. That is, the member mounted in the one direction of the seat unit can be stored inside the outermost shell of the structure. In addition, the one direction is the width direction of the airframe.