A center alignment mechanism of a cavity thickness adjustable mold for continuous casting, comprises a guide mechanism which supports a spindle of a carriage device of each narrow face unit of the mold slidably in the wide face-wise directions of wide face units of the mold, and a driving mechanism which moves the guide mechanism in the narrow face-wise directions of the narrow face unit. The guide mechanism comprises a guide block in which the distal end portion of the spindle is slidably supported, a guide groove formed in the wide face units so as to support the guide block slidably in the narrow face-wise directions of the narrow face unit, a bracket in which the proximal end portion of the spindle is slidably supported, and another guide groove formed in the wide face units so as to support the bracket slidably in the narrow face-wise directions of the narrow face unit. The driving mechanism comprises worm jacks provided between the guide block and the fixed wide face unit and between the bracket and the fixed wide face unit, and a driving source of the worm jacks. Thus, there can be provided a cavity thickness variable mold in which narrow face units are not distorted when they are moved, so that wide face copper plates will not be damaged.