Abstract: A device for indexing the unwound length of a magnetic tape of a cassette, using a two-stage reduction gear which is driven by a reel of the cassette and which is formed of coaxial flat disks or rings nested in one another, two of which carry graduations adapted to pass by a fixed index of the cassette. The first reduction stage includes a ring (127) with an eccentric circular peripheral surface (128) carrying pins for engagement by the reel. The ring (127) is mounted for rotation in a first annular gear (101) with external gear teeth (105). The first annular gear (101) is mounted for oscillation with play about a fixed point. The first reduction stage also includes second annular gear (71) having internal gear teeth which mesh with the external gear teeth (105) of the first annular gear (101) and having an eccentric circular outer peripheral surface (73). The second reduction stage includes a third annular gear (51) with a circular internal surface (52) in which turns with play the second annular gear ( 71).