PURPOSE: To get the upper surface of a seat bottom and that of a laid down seat back at the same level, by a method wherein the upper surface of the seat bottom can be levelled by making an upper and a lower rail into circular arc shape.

CONSTITUTION: Fix the circular arc shaped upper rail 21, keeping its convex side upward, on fixing portions 16a and 16b of a seat frame 16 of a front side seat 10. And fix the circular arc shaped lower rail 24, keeping its convex side upward, on a supporting frame 23 fixed on a motor car floor 22. Assemble the upper rail 21 and lower fail 24 so that free sliding between the two is allowed, and provide a stopping means which can stop the sliding at an appropriate position. When the seat 10 is shifted to the front, the front part of the seat bottom 12 protruding upward falls to almost leveled position, and said continuously leveled surface can be achieved by laying down the seat back 14 backward to the level.