A protective device (14) is attached to a medical instrument (12) provided with a needle (16) and a needle holding part (28), and covers a needle tip (16a) after puncturing of a puncture object by the needle (16). The protective device (14) includes an inner cylinder (36), an outer cylinder (40), and a spring (38), and the outer cylinder (40) receives a cam structure (54) of the inner cylinder (36) and has a guide passage structure (68) configured from one or more guide passages (66). The outer cylinder (40) is provided with a lateral top part (84b) and a distal-end inclined edge part (84d) as resistance force generating parts for causing the inner cylinder (36) to generate a resistance force at a puncture position (92a) or in the vicinity of the puncture position (92a).
