A protection device (14) is attached to a syringe (12) provided with a needle (16). The protection device (14) includes an inner cylinder (44) having a cam structure (66) with at least one protrusion (60)； an outer cylinder (48) that covers the outside of the needle (16) and the inner cylinder (44) ； and a spring (46). The outer cylinder (48) has a guiding path structure (80) including at least one guiding path (82). The guiding path structure (80) includes an elastic member (84) allowing the cam structure (66) to move； and an inner edge (74g) provided to a position in a peripheral wall (72) of the outer cylinder (48) different from the elastic member (84), configured to regulate the movement of the cam structure (66) by engaging with the cam structure (66).
