Provided is a leader block rotary mechanism for a cartridge magnetic tape device, which allows over-rotation of a rotator cam in the capture direction by providing an over-rotation limiting device in a rotator cam so that damages on mechanism parts and motors due to over-rotation of the rotator cam can be prevented. Over-rotation of the rotator cam can be facilitated so that fine adjustment in the rotation amount of the loader cam gear and the rotator cam becomes unnecessary. Further, there is no abnormal capture operation generated even when there is over or less rotation of the rotator cam. The rotator cam is forcibly rotated by a release cam in the release direction through reverse rotation of the loader cam gear so as to prevent the rotation from being stuck at the time of release.