A ball bearing (10, 10a), comprising: an inner ring (20, 20'); an outer ring (30, 30'); a plurality of spherical rolling elements that roll while in contact with the outer ring (30, 30') and the inner ring (20, 20') at contact angles (α), each contact angle (α) corresponding to the outer ring and the inner ring, the outer ring (30, 30') including: an inner annular section having a rolling motion surface with which the rolling elements roll while in contact at said contact angle (α); a thin-wall section extending radially outwardly from the inner annular section in an integrated manner; and an outer annular section (36, 30p1) that is provided on an outer edge section (34e1) of the thin-wall section in an integrated manner so as to support the inner annular section in an axially-movable manner through the thin-wall section; a fluid-supply hole that supplies pressurized fluid to an annular space (R30, SP) partitioned by the inner annular section, the thin-wall section and the outer annular section (36, 30p1); a ring-shaped plug member which closes the annular space (R30, SP) by being inserted into the annular space (R30, SP) while facing the thin-wall section, said plug member being fixed to the outer annular section (36, 30p1) in such a manner as not to permit relative movement; and a seal member made of rubber or resin, said seal member being placed in a gap between said plug member and said inner annular section so that an inner surface of said seal member contacts an outer surface of said inner annular section; the rolling elements being pressed against the outer ring (30, 30')and the inner ring (20, 20') by displacement of the inner annular section in the axial direction, said displacement being caused by elastic deformation of the thin-wall section in the axial direction by the pressurized fluid supplied from the fluid-supply hole, characterized in that the outer ring (30; 30') comprises a single piece consisting of said inner annular section, said outer annular section (36, 30p1) and said thin-wall section, and said gap is arranged so that the plug member is not in contact with the inner annular section. an inner ring (20, 20'); an outer ring (30, 30'); a plurality of spherical rolling elements that roll while in contact with the outer ring (30, 30') and the inner ring (20, 20') at contact angles (α), each contact angle (α) corresponding to the outer ring and the inner ring, the outer ring (30, 30') including: an inner annular section having a rolling motion surface with which the rolling elements roll while in contact at said contact angle (α); a thin-wall section extending radially outwardly from the inner annular section in an integrated manner; and an outer annular section (36, 30p1) that is provided on an outer edge section (34e1) of the thin-wall section in an integrated manner so as to support the inner annular section in an axially-movable manner through the thin-wall section; an inner annular section having a rolling motion surface with which the rolling elements roll while in contact at said contact angle (α); a thin-wall section extending radially outwardly from the inner annular section in an integrated manner; and an outer annular section (36, 30p1) that is provided on an outer edge section (34e1) of the thin-wall section in an integrated manner so as to support the inner annular section in an axially-movable manner through the thin-wall section; a fluid-supply hole that supplies pressurized fluid to an annular space (R30, SP) partitioned by the inner annular section, the thin-wall section and the outer annular section (36, 30p1); a ring-shaped plug member which closes the annular space (R30, SP) by being inserted into the annular space (R30, SP) while facing the thin-wall section, said plug member being fixed to the outer annular section (36, 30p1) in such a manner as not to permit relative movement; and a seal member made of rubber or resin, said seal member being placed in a gap between said plug member and said inner annular section so that an inner surface of said seal member contacts an outer surface of said inner annular section; the rolling elements being pressed against the outer ring (30, 30')and the inner ring (20, 20') by displacement of the inner annular section in the axial direction, said displacement being caused by elastic deformation of the thin-wall section in the axial direction by the pressurized fluid supplied from the fluid-supply hole,