In a packing attachment structure, a packing accommodating recess includes an annular body and an engagement part projecting from the body on a plane of a front face of a flange part and in an one-plane direction of the front face. A packing includes an annular body and a to-be-engaged part projecting from the body. In arrangement, the body of the packing is accommodated in the body of the packing accommodating recess, while the to-be-engaged part of the packing is engaged with the engagement part of the packing accommodating recess.