A rotation detecting device, comprising: a first supporting member disposed around an axial line; a second supporting member situated away from the first supporting member along the axial line, at least one of said first supporting member and said second supporting member being arranged to be rotatable around the axial line, characterized in that, a pair of magnetic field generating portions (21, 22, 23, 24) attached to the first supporting member to face the second supporting member for generating a magnetic field between the first supporting member and the second supporting member, said magnetic field generating portions (21, 22, 23, 24) being arranged around the axial line and away from each other along a circumferential direction around the axial line, one of said magnetic field generating portions (21, 22, 23, 24) having a polarity opposite to that of the other of the magnetic field generating portions; a magnetic field detecting portion (31, 32, 33) attached to the second supporting member to face the first supporting member for detecting the magnetic field generated with the magnetic field generating portions, said magnetic field detecting portion (31, 32, 33) including a magnetic element with a coil wound thereon arranged so that a longitudinal direction of the magnetic element is in parallel to a tangential direction of a circumference around the axial line and over the magnetic field generating portions; a first magnetic member (43, 44, 45) attached to the second supporting member for covering one end portion of the magnetic field detecting portion (31, 32, 33) in the longitudinal direction of the magnetic element; and a second magnetic member (42, 44, 46) attached to the second supporting member for covering the other end portion of the magnetic field detecting portion (31, 32, 33) in the longitudinal direction of the magnetic element, wherein said first magnetic member (43, 44, 45) is arranged to face the second magnetic member (42, 44, 46) at a center portion of the magnetic field detecting portion (31, 32, 33) with a space in between. a first supporting member disposed around an axial line; a second supporting member situated away from the first supporting member along the axial line, at least one of said first supporting member and said second supporting member being arranged to be rotatable around the axial line, characterized in that, characterized in that, a pair of magnetic field generating portions (21, 22, 23, 24) attached to the first supporting member to face the second supporting member for generating a magnetic field between the first supporting member and the second supporting member, said magnetic field generating portions (21, 22, 23, 24) being arranged around the axial line and away from each other along a circumferential direction around the axial line, one of said magnetic field generating portions (21, 22, 23, 24) having a polarity opposite to that of the other of the magnetic field generating portions; a magnetic field detecting portion (31, 32, 33) attached to the second supporting member to face the first supporting member for detecting the magnetic field generated with the magnetic field generating portions, said magnetic field detecting portion (31, 32, 33) including a magnetic element with a coil wound thereon arranged so that a longitudinal direction of the magnetic element is in parallel to a tangential direction of a circumference around the axial line and over the magnetic field generating portions; a first magnetic member (43, 44, 45) attached to the second supporting member for covering one end portion of the magnetic field detecting portion (31, 32, 33) in the longitudinal direction of the magnetic element; and a second magnetic member (42, 44, 46) attached to the second supporting member for covering the other end portion of the magnetic field detecting portion (31, 32, 33) in the longitudinal direction of the magnetic element,