A protective device, for covering a needle tip of a needle of a medical instrument after puncturing of a puncture object by the needle, includes an inner cylinder, an outer cylinder, and a spring. The outer cylinder receives a cam structure of the inner cylinder and has a guide passage structure that includes one or more guide passages. The outer cylinder is provided with a lateral top part and a distal-end inclined edge part as resistance force generating parts for causing the inner cylinder to generate a resistance force at a puncture position or in the vicinity of the puncture position.