A spindle for transmitting a rotational driving force to a mill roll, comprising: a female coupling part having at one end side an engagement recess capable of receiving an engagement projection of a male coupling part; and a rotary joint including an annular rotor part disposed at another end side of the female coupling part and configured to rotate together with the female coupling part, and an annular stator part disposed on an outer peripheral side of the annular rotor part, wherein the female coupling part includes at the one end side a pair of protruding portions (34A, 34B) each of which has a facing surface defining the engagement recess, wherein the female coupling part has a plurality of oil supply passages (90, 92) for supplying oil to the engagement recess, wherein the rotary joint has a plurality of annular oil passages each of which communicates with any of the plurality of oil supply passages, wherein the plurality of oil supply passages (90, 92) includes at least one oil supply passage disposed corresponding to each of four areas (A1-A4) obtained by dividing the engagement recess by a center line between the pair of facing surfaces and a straight line perpendicular to the center line, in a cross-section perpendicular to an axial direction of the spindle, and characterised in that the at least one oil supply passage (90, 92) disposed corresponding to one area of the four areas communicates with, among the plurality of annular oil passages, an annular oil passage different from an annular oil passage with which the at least one oil supply passage disposed corresponding to an area adjacent to the one area of the four areas communicates. a female coupling part having at one end side an engagement recess capable of receiving an engagement projection of a male coupling part; and a rotary joint including an annular rotor part disposed at another end side of the female coupling part and configured to rotate together with the female coupling part, and an annular stator part disposed on an outer peripheral side of the annular rotor part, wherein the female coupling part includes at the one end side a pair of protruding portions (34A, 34B) each of which has a facing surface defining the engagement recess, wherein the female coupling part has a plurality of oil supply passages (90, 92) for supplying oil to the engagement recess, wherein the rotary joint has a plurality of annular oil passages each of which communicates with any of the plurality of oil supply passages, wherein the plurality of oil supply passages (90, 92) includes at least one oil supply passage disposed corresponding to each of four areas (A1-A4) obtained by dividing the engagement recess by a center line between the pair of facing surfaces and a straight line perpendicular to the center line, in a cross-section perpendicular to an axial direction of the spindle, and characterised in that the at least one oil supply passage (90, 92) disposed corresponding to one area of the four areas communicates with, among the plurality of annular oil passages, an annular oil passage different from an annular oil passage with which the at least one oil supply passage disposed corresponding to an area adjacent to the one area of the four areas communicates.