PURPOSE: To perform simple modification of an existing bicycle to a motorcycle by a method wherein a drive unit to contain a detecting part, a control part, and a drive part in one case is removably installed to a rear wheel frame and a source unit is removably arranged in an opposite position thereto.

CONSTITUTION: A drive auxiliary unit 1 is constituted such that a detecting part 3, a control part 4, and a drive part 5 are contained in a unit case 6, and removably installed on the installation side of the drive chain 31 of rear wheel frames 32 and 33 by means of a mounting jig 7. The source unit of a drive auxiliary unit 1 is removably installed to a mounting jig and in a position on the opposite side to a drive auxiliary unit 1 for rear wheel frames 32 and 33 by means of a mounting jig. The magnitude of the tension of the drive chain 31 is detected by the detecting part 3 and converted into an electric signal, which is outputted. An electric signal is processed by the control part 4, and when it is decided that drive auxiliary is needed, a control signal is outputted to a drive part 5 and the drive part 5 to receive the control signal is driven by a chain sprocket 35 to perform drive auxiliary.