An insertion tool includes a drive power transmitting mechanism disposed on a proximal-end portion of an insertion portion, for transmitting drive power to a driven member disposed on a distal-end side of the insertion portion. The drive power transmitting mechanism includes a gear train for transmitting drive power from a drive source that generates the drive power, and a joint mechanism for joining the drive source to the gear train to transmit the drive power from the drive source to the gear train and unjoining the drive source from the gear train.