Abstract: A system for automatically assembling a tape cassette including upper and lower cassette housing halves, a pair of reel hubs received in said housing halves and a magnetic tape wound about both reel hubs, which comprises a magnetic tape-loading mechanism for winding a magnetic tape about the reel hubs and loading the reel hubs and magnetic tape into the lower housing half placed on an assembly line, wherein the magnetic tape-loading mechanism comprises a reel hub-fitting device for fitting each of the paired reel hubs on a tape takeup jig; a tape takeup device for winding a magnetic tape about the reel hubs; and a tape transfer device for holding the reel hubs fitted on the tape takeup jigs and a magnetic tape in the same arrangement as actually takes place in the tape cassette and thereafter loading both reel hubs and magnetic tape into the lower housing half.