To provide a simple device so constructed that a door is locked by magnetized force and a shock following opening and closing of the door is buffered by magnetic force.

Contact surfaces 1a, 2a of a sliding door 1 traveling on a door sill and a rail and a post 2 where the sliding door abuts are formed so that two plate magnets 3, 10 are opposite to each other with the same polarity, one magnet 3 is fixed to one contact surface 2a, and the other magnet 10 is seated in a case 12 as a movable magnet 10 capable of sliding upward, and installed on the other contact surface 1a with the center position 10a of the movable magnet 10 displaced to the upper side relatively to the center position 3a of the fixed magnet 3. This device can be intalled also on a contact surface between an opening door opened and closed through a hinge and a wall on which the door is opened to abut.