PURPOSE: To mount or dismount a screw cap without a fail even when the surface of a drum deforms, by a method wherein two or more supporting rods are attached vertically on a unit plate which is able to move down on the drum, the lower ends of the supporting rods provided with compression springs are inserted through a suspension plate and a screw cap sticking head is mounted on a chuck spindle.

CONSTITUTION: Compression springs 58 are mounted between a unit plate 55 and a suspension plate 57 so as to always press the suspension plate 57 down. Large holes 59 having larger holes than the diameter of supporting rods 56 are provided on the rod paths of the suspension plate 57, and washers 60 having conical receiving surfaces 60a are supported on the lower ends of the supporting rods 56 and seated on the large holes 59 from behind. Between a motor base 61 and the suspension plate 57, a chuck spindle 62 extending vertically is supported free to rotate on a bearing 63, and a driving pinion 66 of a motor 65 is engaged with a driven plane gear 64 that is attached on the middle portion of the chuck spindle. A screw cap sticking head 67, that is made of rubber and sticks to the surface of a screw cap Y, is fixed on the lower end of the chuck spindle 62.