rotated downwards so as to prevent the three and fourth gear teethed parts 1130 and 1330 from being disengaged from each other when the second one way stoppers 1200 move in the reverse direction the rotating members 1220 rotate upwards thus moving the fourth bracket 1100 away from the fifth bracket 1300 thereby enabling the height adjustment apparatus to be contracted in other words the height adjustment apparatus of the present invention is extended by a degree with which the user applies force to the upper board 1102 upwards and then it is retained at that height when the upper board 1102 is lifted to the uppermost height the three and fourth gear teethed parts 1130 and 1330 are disengaged from each other thus allowing the height adjustment apparatus to be contracted by its own weight meanwhile the height adjustment apparatus of the second embodiment may be constructed such that the positions of the fourth bracket 1100 and the fifth bracket 1300 are switched that is the fifth bracket 1300 having the second on way stoppers 1200 may be disposed at the position of the fourth bracket 1100 and the fourth bracket 1100 may be disposed at the position of the fifth bracket 1300 in this case the sixth bracket 1400 is coupled to the rear side of the fourth bracket 1100 that is changed in position the operation of this modified construction is the same as.