A medical-device force transmission mechanism transmitting a force input at the proximal end of an elongated insertion portion inserted into a body and acting along the longitudinal direction of the insertion portion, to a rotation unit supported at the distal end of the insertion portion in a rotatable manner about the longitudinal axis of the insertion portion, the medical-device force transmission mechanism including: a first force transmitting part passing through the insertion portion and transmitting the force to the distal end of the insertion portion; a force converting part disposed at the distal end of the insertion portion and converting the force transmitted by the first force transmitting part into a rotational force; and a second force transmitting part transmitting the rotational force converted by the force converting part to the rotation unit at a different rotational speed.