To provide a relocking mechanism of a safe door, capable of prolonging the attacking time of the safe door and informing abnormality early.

This relocking mechanism of the safe door to which a cylinder lock is attached includes a hole closing part for blocking a cylinder hole forcibly when the cylinder lock is shot through the inside of the safe door. The hole closing part includes a shutter blocking the cylinder hole to prevent attack from the outside and a shutter guide for guiding the shutter to block the cylinder hole when the cylinder hole is shot through. When the cylinder lock is shot through, the shutter is not supported any more and is guided by the shutter guide to block the cylinder hole. The shutter guide has a shutter support for supporting the shutter to prevent it from being deviated while the cylinder hole is blocked. This relocking mechanism has a control unit for giving an alarm when detecting the movement of a relocking shaft.