A seat device that can reliably regulates a reclining operation when in a long state where a back of a seat is along a wall. The seat device (10) that can convert the state of the seat (1) includes a reclining mechanism (50) that can tilt a backrest (3) of the seat (1), wherein an operation of tilting the backrest (3) by the reclining mechanism (50) is disabled when the seat (1) is in the long state.