A medical treatment instrument of the present invention includes: a pair of opening and closing pieces； an opening and closing shaft section configured to hold the pair of opening and closing pieces so that the pair of opening and closing pieces are able to pivot relative to each other； a first gear fixed to either of the pair of opening and closing pieces to pivot about the opening and closing shaft section； a second gear which is able to rotate to transmit a rotational force to the first gear； a rotatable member fixed to the second gear coaxially with the second gear and having a diameter larger than a diameter of the second gear； and a power transmission member provided around an outer circumference of the rotatable member and configured to rotate the rotatable member.
