To provide a theft preventive device for a bicycle which requires no attachment to independent dedicated locking means and no consideration of storage of the locking means.

This theft preventive device is constituted of a handle bar member 8 fitted on a saddle-height adjusting bolt (screw member for screwing component member of bicycle) 7 so as to be capable of being inserted and taken-out, an inserting member (inserting part of handle bar member) 14 formed on one of a pair of right and left seat stays (body frames) 11 crossing a rear wheel (wheel) 13, and a key member (locking mechanism) 15 formed at the other seat stay 11. The rotation of the rear wheel 13 is locked by inserting the handle bar member 8 into the inserting member 14 and the rear wheel 13, and fixing its end part by the key member 15. In this case, an existing handle bar member 8 itself which is one of component parts for the bicycle is used as a locking means, therefore, it is possible to eliminate an independent dedicated locking means such as a U-lock and a chain, and make it unnecessary for a user to consider storage of the locking means.