Abstract: An object of the present invention is to efficiently provide a high-quality rare-earth iron garnet single crystal. The invention relates to a rare-earth iron garnet single crystal substantially composed of an Re3Fe5-xMxO12 single crystal (where Re is at least one element selected from Y, Bi, Ca, and lanthanide rare-earth elements with atomic numbers of 62 to 71; M is at least one element selected from Al, Ga, Sc, In, Sn and transition metal elements with atomic numbers of 22 to 30; and 0≦x<5), with the number per unit surface area (grains/cm2) of crystal grains that form low-angle tilt boundaries equal to 0≦n≦102; and also relates to a device in which this rare-earth iron garnet single crystal is used.