PURPOSE: To easily adjust the full open position of a door and facilitate the loading/unloading of baggage in the back door of an automobile by joining the end part of a piston rod and the end part of an outer cylinder via a screwing means and engaging an engaging part with a stopper part.

CONSTITUTION: At the maximumly expanded time of a damper stay A, the stopper part 11 of an outer cylinder 9 is moved axially leftward by one stroke S and engaged with the engaging part 12 of a cylinder 1 making a piston rod 3 reach the maximumly expanded position. And, by adjusting the axial position of the piston rod 3 with respect to the outer cylinder 9 by the rotation of the outer cylinder 9, the maximumly expanded dimension L of the damper stay A is adjusted within a range R. Thereby, the full-open position of a door can be easily adjusted in accordance with the necessity of an operator.