PURPOSE: To certainly fix a seat at a desired position and prevent its movement in a slide type driver's seat device of a construction machine furnished with a sliding mechanism and a positioning mechanism of the seat.

CONSTITUTION: A latch groove 6b of a latch plate 6 to fit a positioning claw 8 which is a constitutional element of a positioning mechanism is formed in a tapered shape with an opening end the width of which is enlarged by setting width W3 of a groove bottom smaller than width of the positioning claw 8. The latch plate 6 is fixed on a slidable rail 5 installed on a seat, and the slidable rail 5 is supported by a guide rail 1 set on a floor of a cabin.