A connecting structure includes: a cylindrical part formed in a rotation operation shaft; a shaft part formed in an operation knob; and division grooves respectively extending in the axial direction of the cylindrical part, wherein: the cylindrical part and a shaft part are tight fit connected together; in two or more portions of the cylindrical part at equal angle intervals in the peripheral direction thereof, there are formed the division grooves, thereby being able to enhance the flexural deformation property of the cylindrical part as well as to secure mutual concentricity between the cylindrical part and the shaft part formed of resin; and at least one of the division grooves is used as a key groove with which is fitted a key portion formed in the outer peripheral portion of the shaft part, thereby preventing the cylindrical and shaft parts from rotating with respect to each other.