PURPOSE: To always ensure the optimum contact pressure by controlling the turning of one of two bearings for supporting both ends of a torsion spring for pressing an outer roller to the wheel envelope in a roller dynamo having the outer roller coming into contact with the wheel envelope of a bicycle.

CONSTITUTION: A roller dynamo includes a roller 1 pivoted by a ball bearing 2, and an electromagnet 3, a stator 4 and an electromagnetic coil 5 are disposed in the roller 1. The roller 1 is adapted to freely rotate about a stator shaft 6 held by coupling arms 7 on both sides. Two coupling arms 7 are turned on a rotating shaft 8 to separate the roller 1 from the wheel envelope. A helical torsion spring 14 is incorporated in the cylinder 9 of the rotating shaft 8. In this case, one end of the spring 14 is fitted to non-rotation bearing 15, and the other end thereof is fitted to a bearing 16 provided with an external tooth 17, and after the engagement of the external tooth 17 with an internal tooth 18 is separated, the bearing 16 is turned to control the torsional force of the spring 14.