An aircraft seat unit comprising: a lower structure (240, 300) of the aircraft seat unit; and three fixtures (272, 274, 276) configured to attach the lower structure (240, 300) directly to a pair of seat tracks (282, 284) of the aircraft, wherein the lower structure includes a seat base formed in a hollow box shape using integrally formed members which are formed from thermoplastic resins or composite materials and that define a hollow region; a bottom of the seat base comprises a flat inner surface on the hollow region side and an opposite outer surface, provided with recesses, between which mounting blocks, particularly a first mounting block, a second mounting block and a third mounting block, are inserted; the three fixtures (272, 274, 276) include a lateral position adjustment member (296, 297, 298) configured to laterally adjust a fitting position with the pair of seat tracks, a first fixture is fixable to one of the pair of seat tracks and is disposed below the first mounting block, a second fixture is fixable to the one of the pair of seat tracks at a point separated from the first fixture and is disposed below the second mounting block, and a third fixture is fixable to another one of the pair of seat tracks at a corresponding portion between the first fixture and the second fixture and is disposed below the third mounting block. a lower structure (240, 300) of the aircraft seat unit; and three fixtures (272, 274, 276) configured to attach the lower structure (240, 300) directly to a pair of seat tracks (282, 284) of the aircraft, wherein the lower structure includes a seat base formed in a hollow box shape using integrally formed members which are formed from thermoplastic resins or composite materials and that define a hollow region; a bottom of the seat base comprises a flat inner surface on the hollow region side and an opposite outer surface, provided with recesses, between which mounting blocks, particularly a first mounting block, a second mounting block and a third mounting block, are inserted; the three fixtures (272, 274, 276) include a lateral position adjustment member (296, 297, 298) configured to laterally adjust a fitting position with the pair of seat tracks, a first fixture is fixable to one of the pair of seat tracks and is disposed below the first mounting block, a second fixture is fixable to the one of the pair of seat tracks at a point separated from the first fixture and is disposed below the second mounting block, and a third fixture is fixable to another one of the pair of seat tracks at a corresponding portion between the first fixture and the second fixture and is disposed below the third mounting block. the three fixtures (272, 274, 276) include a lateral position adjustment member (296, 297, 298) configured to laterally adjust a fitting position with the pair of seat tracks, a first fixture is fixable to one of the pair of seat tracks and is disposed below the first mounting block, a second fixture is fixable to the one of the pair of seat tracks at a point separated from the first fixture and is disposed below the second mounting block, and a third fixture is fixable to another one of the pair of seat tracks at a corresponding portion between the first fixture and the second fixture and is disposed below the third mounting block.