To provide a connecting structure of pipes capable of easily connecting pipes without special equipment or material and surely cutting off water.

This connecting structure of pipes is characterized by comprising a male screw part 20 formed on one end with a protruding part 22 on the periphery which abuts on the inside of the male screw part 20, and a female screw part 30 engageable with the male screw part 20, formed on the other end with a socket part 32 which abuts on the outside of the female screw part 30 and contacts the protruding part 22. Connection is made by engaging one pipe with another. The protruding part 22 increases the diameter of the socket part 32 by using force generated by screwing and the socket part 32 decreases the diameter. The increased diameter of the socket part 32 decreases due to elasticity and comes into close contact with the protruding part 22, and likewise, the decreased diameter of the protruding part 22 increases to be close contact with the socket part 32 to surely cut off water.