A protection device is attachable to a syringe that includes a needle. The protection device includes an inner cylinder having a cam structure with at least one protrusion； an outer cylinder that covers the inner cylinder and the needle； and a spring. The outer cylinder has a guiding path structure including at least one guiding path. The guiding path structure includes an elastic member allowing the cam structure to move； and an inner edge provided at a position in a peripheral wall of the outer cylinder different from the elastic member. The inner edge is configured to regulate movement of the cam structure by engaging with the cam structure.
