To keep a seat removably fixed to a seat base by a fixing member out of the way of crew wherein the seat does not obstruct passing of the crew.

A seat base 28 is provided at the position corresponding to the side of a floor panel 13 in the direction of width of a ship body and supports a seat 29 on the top surface thereof. A crew passage 34a is made on the floor panel 13 at the position which is opposite to a bulwark in the direction of width of the ship body with respect to the seat base 28 and is adjacent to the seat base 28. A fixing member 56 for removably fixing the seat 29 to the seat base 28 is made near the front part and/or the rear part of the seat 29 and in the middle of the direction of width of the seat 29.