To provide a connector which surely connects tubes with fewer components and in a simple structure, and also is reduced in outer diameter.

The connector 10 connects a first tube 1 having an annular protrusion 1a, and a second tube, and includes: a body 20 having a connecting part 21 and an inserting part 23; an O ring 60; and a retainer 40. The retainer 40 has: a body part 41 controlling the movement of the O ring 60; and an engagement piece 47 arranged continuing from the body part 41 via a slit 45, and having an engagement claw 53 formed at the end 51 thereof. An opening 31 is formed in the body 20, and the end 51 of engagement piece 47 of the retainer 40 is inserted into the opening 31. Then, the engagement piece 47 can be deflected outward from the outside of the body 20.