In a stencil printing machine, when a printing drum is set at its operating position in the direction of its axis, a drum switch is turned on, and a locking piece is engaged with a groove formed in an engaging rod integral with the printing drum, so that a lock switch is also turned on. In the case where the printing drum pushed into the printing machine is not at the operating position, the drum presence/absence switch is turned on, and the locking piece is not engaged with the groove, so that the lock switch is off. In this case, the locking piece is reciprocated a predetermined distance in both directions. As a result, the locking piece is engaged with the groove of the engaging rod; that is, the printing drum is moved axially to the operating position.