To provide an assisting device capable of easily and quickly mounting and positioning a front transmission.

When a front transmission is mounted and positioned on a frame of a bicycle, a disengagemable assisting member 21 consisting of the first and second elements 22, 23 is engaged with the transmission, and the first element 22 is inserted between two levers 10, 11 of a parallel square link device of the transmission. A link device is held on an optimum mounting position against the action of a return spring 18, the second element 23 is mounted in a guide fork member 5 of the transmission, and a reference surface is contacted with a peripheral edge of a gear to fix the guide fork member on a proper position while radially separated from the gear of large diameter by a proper distance.