A fitting includes a screw portion that is rotatable around an axis extending in one direction, an operating portion that applies a torque around the axis to the screw portion, and a rotation auxiliary member that is configured to be engageable with the operating portion. A slot extending from a first end to a second end is provided in the rotation auxiliary member, and also a first projection that protrudes outward is provided on the rotation auxiliary member. A positional relationship of the first projection is set such that an engaging force between the rotation auxiliary member and the operating portion is increased due to a change in width of the slot in a case where a force is applied to the first projection in the first rotation direction with the rotation auxiliary member engaging with the operating portion.