To provide a bicycle alarm device of a simple structure which can be easily mounted, wherein a burden is not imposed on a bicyclist.

A bicycle alarm device is used by being installed on a bicycle that has a blade and the wheel supported by the blade. The device includes: a contact part; a contacted part installed in the contact part in a contactable manner; a casing that accommodates the contact part and the contacted part; and a roller that extends from the casing and is arranged to contact the tire and rotate. The rotation of the roller makes the contact part rotate and the contact sound is generated by contact of the contact part and the contacted part by the rotation.