PURPOSE: To permit a door to be kept in a state of being opened by a method in which a turnable coupling claw is provided in a part to be inserted into an outer cylinder in a sliding manner, and a recession where the coupling claw runs idle is formed on the coupling teeth row to connect the coupling law with the outer cylinder and both ends of the coupling teeth row.

CONSTITUTION: A coupling claw 5 is turnably attached to the basal end of an insert part 2, and a pin 7 on the base side wall is coupled with the spring 6 of the claw 5. On the side wall 8 of an outer cylinder 1, a continuously repeated coupling teeth row 9 is provided, and a recession 10 is formed on both ends of the row 9. When the part 2 is moved to the direction of arrow A, it goes out while striking the tip of the claw 5 by the top 11 of the teeth 9. When extending operation is stopped, the claw 5 comes into contact with the teeth 9 on the corresponding position and is locked. In unlocking operation, when the part 2 is extended maximumly, the claw 5 separates from the top 11 and goes to the bottom 10, where it turns to the direction of arrow C for unlocking. When the part 2 is pulled into the cylinder 1 and stopped at the most contracted position, it is reversely turned on the recession 10 of the end position and allowed to be pulled out gain.