PURPOSE: To enable mounting of seat members on both high and low positions by providing a seat section having a hatch between the deck face and floor face and making a seat member comprised of a seat face section to be used as a hatch cover and a foot wall section mountable/dismountable.

CONSTITUTION: A stepped seat section 4 is formed higher than the floor face 12 between the deck face 11 and the floor face 12. A hatch 52 is formed in the upper face of seat section 4 to make a stepped seat member 6 comprised of a seat face section 65 to be used as a cover for the hatch 52 and a foot wall section 62 along the front wall 41 of the seat section 4 mountable/dismountable on the seat section 4. Consequently, the seat member 6 can be mounted at both high and low positions of the seat section 4 to enable selection of seat position between high and low positions.