PURPOSE: To permit a door to be kept in a state of being opened on the way of opening by a method in which as insert part to be inserted into an outer cylinder in a sliding manner, a coupling claw rotatably attached to the outer cylinder, a coupling teeth row formed in the insert part, and a recession where the claw can turn on both ends of the teeth row are provided.

CONSTITUTION: A coupling claw 5 is rotatably attached to the side wall of an outer cylinder 1, and a spring 6 to be operated to the direction of coupling with a coupling tooth 9 is attached to the pin 4 of the claw 5. A coupling teeth row with continuous coupling teeth 9 is formed on the side wall 8 of an insert part 2, and a recession 10 where the tip of the claw 5 can turn is formed on both ends of the row. When the part 2 is pulled out and moved to the direction of arrow A, it slides while the tip of the claw 5 by the top of the teeth 9, and when extending operation is stopped, the claw 5 comes into contact with the teeth 9 on the corresponding position for locking. In unlocking operation, when the part 2 is extended maximumly, the claw 5 separates from the top 11 and turns to the direction of arrow C in the recession 10 on coth ends of the part 2 for unlocking. The part 2 becomes contracted.