A groove portion depressed from an unit outer surface toward a first perpendicular direction extends over an entire length of a transmission unit in axially parallel directions, and in the transmission unit, a sectional of a first unit component perpendicular to a longitudinal axis is a first sectional shape and a sectional shape of a second unit component perpendicular to the longitudinal axis is a second sectional shape smaller in sectional area than the first sectional shape. The sectional area changing portion changes from the first sectional shape to the second sectional shape so that a gravity center position in a case where the longitudinal axis is a reference position is consistent over the entire length of the transmission unit in the axially parallel directions.
