A driving force transmission mechanism for medical devices, includes a tubular portion including: an opening edge portion which allows a support portion of a drive shaft to be supported at an output end in a state where at least a part of the output end is accommodated in the tubular portion； and a first adjustment section which is configured to fix a gear train in a state where the drive shaft is inserted into the tubular portion and a distal portion of the drive shaft is arranged at a predetermined position where a driven portion is able to be driven and a state where a position of the output end of the gear train is adjusted relative to a position of the support portion of the drive shaft.
