To provide a screw locking device capable of easily manufacturing at a low cost and preventing an aged loose of a screw thread portion in a thread.

In the screw locking device locking a screw portion 2 in a screw thread 22, the screw thread 22 is formed in a structure portion 1. A screwed screw thread 20 is formed in a screw portion 3, and the screwed screw thread is fitted with the screw thread 22 formed in the structure portion 1. A safety sleeve 5 surrounds the screw portion 3, and the safety sleeve 5 can be fixed to the structure portion 1 in a rotative direction of the screw thread 22. A recessed portion 16 is formed in the screw portion 3, and the safety sleeve 5 can be pushed and formed into the recessed portion in the radial direction. Thus, the safety sleeve 5 is locked on the screw portion 3 in a rotative direction of the screwed screw thread 20.