The device (10) for selective access closure inside a catheter comprises a conduit (14) having a rear extremity (16) for the insertion of a medical instrument and a mechanism (27) for adjusting the cross-section provided within the conduit (14). The mechanism (27) comprises a member (32) for repositioning the device between an opening movement and a closing movement. The direction of movement of the repositioning member (32) along both the closing path and the opening path are identical.