PURPOSE: To provide a liquefied high pressure gas cylinder cap fitting/removing method enabling the fitting/removal of a cap at the gas charging time without manual help.

CONSTITUTION: Clockwise-rotated roller type clamp mechanism 21 provided in the temporary stop position of a liquiefied high pressure gas cylinder 200 in a carry-in path A to a rotary charger, with its clamping face formed of a drive roller 8 and a driven roller is lowered to clamp a screw-in type cap 204 at the upper end part of the cylinder 200, and the drive roller 8 is rotated clockwise to remove the cap 204 from the cylinder 200. The cap 204 removed from the cylinder 200 is indirectly received by counterclockwise-rotated roller type clamp mechanism provided in the elevatingly movable state on the vertical line of a cap receiving part, with its clamping face formed of a drive roller and a driven roller, and this cap 204 is screwingly fitted into the screwed part 205 of the cylinder 200 after the fixed quantity charging of gas.