A fastener device (10) for fastening a load to a rail (4) that is provided with a slideway (4′) defined by a bottom wall (5), two side walls (6), and a top wall (7) including a succession of wide slots (8) and of narrow slots (9), said fastener device (10) comprising a base (20) and two shoes (30) suitable for being inserted in said wide slots (8) and for co-operating with the top wall (7) in a narrow slot (9) by shape interference. The device further comprises first and second slides (31, 32), each including one of said two shoes (30), spacer means (40) for spacing apart said first and second slides (31, 32), detector means (50) for detecting a rail (4′), approach means (60) for causing said first and second slides (31, 32) to approach each other, and blocking means (70) for longitudinally blocking said base (20).