A connection structure is provided having a female thread formed on the inner peripheral surface of a lock ring in a male connector engaging a male thread on the outer peripheral surface of a female connector when the male connector and the female connector are screwed to each other. Further, protrusion strips are formed on the rear side portion of the male thread on the outer peripheral surface of the female connector, and the notches that can engage with the protrusion strips are set on the tip portion of the lock ring. As a result, when the male thread and the female thread are screwed to each other, the screwed state is appropriate when the protrusion strips and notches are engaged with each other.