Tube fitting and tube fitting structure

A tube fitting comprising:

• an outer pipe;
• an inner pipe which is coaxially inserted in the outer pipe, and has such an outer diameter that gives a space between an outer surface of the inner pipe and an inner surface of the outer pipe, in which space a tube member to be joined is inserted;
• and
• a sealing means disposed between the outer pipe and the inner pipe, wherein:
• the sealing means is configured to seal a clearance between the tube member inserted in the space and the outer pipe or the inner pipe;
• the outer pipe, the tube member and the inner pipe are fixedly engaged by pressing;
• the outer pipe extends outwardly in its axial direction further than an end of the inner pipe;
• a length:
• L (mm) of extension of the outer pipe from the end of the inner pipe and an outer diameter:
• D (mm) of the tube member satisfy the following formula:
• L≥0.4×D−6.25 (mm);
• a stopper is formed integrally on the outer surface of the inner pipe, the stopper being contacted with an end of the tube member so as to determine a length of insertion of the tube member;
• the stopper is a ring-shaped projection continuously extending in a circumferential direction of the inner pipe, the ring-shaped projection being integrally formed on an outer circumferential surface of the inner pipe with a height not to contact with an inner circumferential surface of the outer pipe;
• at least one cutout crossing the ring-shaped projection is formed;
• and
• the end of the tube member is contacted with the at least one cutout, so that clearances located inside and outside the tube member are allowed to communicate with each other through the at least one cutout.