A cage (14, 24) for holding a rolling element (13, 23) in a rolling bearing (1, 2), the cage (14, 24) being made of a magnesium alloy, the cage (14, 24) being molded by injection molding, characterized by a confluence region being brought away to outside the cage (14, 24) during said injection molding, said confluence region being a region including a void formed by merging of flows of said magnesium alloy being in a complete molten state, the mold used for said injection molding comprising: a cavity portion (61, 61 A, 61B) having a shape corresponding to that of said cage (14, 24); and a runner portion connected to said cavity portion (61, 61A, 61B) and including a gate portion supplying said magnesium alloy to said cavity portion (61, 61A, 61B), said cavity portion (61, 61A, 61B) including a weld region, which is a region at which flows of said magnesium alloy supplied from said gate portion of said runner portion to said cavity portion (61, 61A, 61B) are merged, said mold further comprising an overflow portion connected to said weld region of said cavity portion (61, 61A, 61B) and storing said magnesium alloy having reached said weld region and flooded from said cavity portion (61, 61 A, 61B), said runner portion has a smaller cross sectional area in a cross section perpendicular to the longitudinal direction thereof, as it comes closer to cavity portion (61, 61A, 61B), and has the smallest cross sectional area at the gate portion boundary surface which is a surface of a boundary with cavity portion (61, 61A, 61B), said overflow portion has a smaller cross sectional area in a cross section perpendicular to the longitudinal direction thereof, as it comes closer to cavity portion (61, 61A, 61B), and has the smallest cross sectional area at the discharging portion boundary surface which is a surface of boundary with cavity portion (61, 61 A, 61B). a cavity portion (61, 61 A, 61B) having a shape corresponding to that of said cage (14, 24); and a runner portion connected to said cavity portion (61, 61A, 61B) and including a gate portion supplying said magnesium alloy to said cavity portion (61, 61A, 61B), said cavity portion (61, 61A, 61B) including a weld region, which is a region at which flows of said magnesium alloy supplied from said gate portion of said runner portion to said cavity portion (61, 61A, 61B) are merged, said mold further comprising an overflow portion connected to said weld region of said cavity portion (61, 61A, 61B) and storing said magnesium alloy having reached said weld region and flooded from said cavity portion (61, 61 A, 61B), said runner portion has a smaller cross sectional area in a cross section perpendicular to the longitudinal direction thereof, as it comes closer to cavity portion (61, 61A, 61B), and has the smallest cross sectional area at the gate portion boundary surface which is a surface of a boundary with cavity portion (61, 61A, 61B), said overflow portion has a smaller cross sectional area in a cross section perpendicular to the longitudinal direction thereof, as it comes closer to cavity portion (61, 61A, 61B), and has the smallest cross sectional area at the discharging portion boundary surface which is a surface of boundary with cavity portion (61, 61 A, 61B).