To provide a tube fitting, in which an accident wherein a flexible pipe is separated due to a fastening failure caused by thermal deformation of a fastening ring can be prevented even if the fastening ring is formed of a productive synthetic resin.

A cap nut includes a fastening ring diameter-contracted state regulation part having a shape in which part of the fastening ring contracted in diameter to the nipping completion state of the flexible part by fastening of the cap nut can be fitted with its outer circumferential surface being in close contact with the inner circumferential surface of the cap nut. A cap nut screwing male thread cylindrical part includes a cutout part formed at the top end thereof, the cutout part having a shape in which the residual part of the fastening ring contracted in diameter to the nipping completion state of the flexible pipe by fastening of the cap nut is fitted with its outer circumferential surface being in close contact with the inner circumferential surface of the cap nut screwing male thread cylindrical part.

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