An apparatus for automatically imprinting several lines of characters on the rounded head of each steel or like metal cylinder for a compressed gas. The apparatus includes work rest means arranged between loading and unloading platforms and adapted to permit a gas cylinder which has been transferred from the loading platform to rest horizontally thereon so as to be rotatable about its own axis. Several marking mechanisms movable relative to the work rest means each include an arcuate marking surface on which a line of characters is formed in relief. The marking surface of each mechanism is successively forced into contact with the gas cylinder on the work rest means, and the marking mechanism is then rotated in rolling contact with the gas cylinder to imprint the line of characters thereon. The gas cylinder that has been marked with the desired lines of characters is transferred onto the unloading platform.