A seat unit for an aircraft that can be directly fixed at three points to a seat track provided on the floor of the aircraft. One representative seat unit and attachment method thereof relates to a seat unit comprising a lower structure of the seat unit and three fixtures configured to attach the lower structure directly to a pair of seat tracks of a mobile body, wherein the three fixtures include a lateral position adjustment member configured to laterally adjust a fitting position with the pair of seat tracks, a first fixture is fixed to one of the pair of seat tracks, a second fixture is fixed to one of the pair of seat tracks at a point separated from the first fixture, and a third fixture is fixed to another one of the pair of seat tracks at a corresponding portion between the first fixture and the second fixture.