A magnetic encoder manufacturing method for manufacturing, using an injection molding mold, a magnetic encoder which includes an annular support member formed from a steel plate, and an annular plastic magnet, and has no adhesive layer between the annular support member and the annular plastic magnet, the annular support member includes a cylindrical portion, and a flange portion extending outward in a radial direction from an end edge of the cylindrical portion, the annular plastic magnet being formed in a shape having, on an outer circumferential side thereof, a turning molded portion covering an outer circumferential part of the annular support member so as to turn around toward an inner diameter side, a difference between an outer diameter of the annular plastic magnet and an outer diameter of the annular support member is set to be not less than 0.3 mm a gate of the injection molding mold being a disk gate located on an inner diameter side of the annular plastic magnet, the disk gate is located outwardly relative to a curved portion between the cylindrical portion and the flange portion of the annular support member, a length L of the disk gate in an axial direction being in a range of 0.2 mm ≤ L ≤ 0.6 mm, mechanical characteristics of a material of the annular plastic magnet being set such that a tensile strength is 65 MPa or more, and a Young's modulus is 4000 MPa or more and 15000 MPa or less, the method comprising: opening the injection molding mold and placing the annular support member as an insert object in the injection molding mold; and closing the injection molding mold and injecting the material of the annular plastic magnet that is melted, into a cavity in the injection molding mold through the disk gate, to perform injection molding of the annular plastic magnet having the shape. the annular support member includes a cylindrical portion, and a flange portion extending outward in a radial direction from an end edge of the cylindrical portion, the annular plastic magnet being formed in a shape having, on an outer circumferential side thereof, a turning molded portion covering an outer circumferential part of the annular support member so as to turn around toward an inner diameter side, a difference between an outer diameter of the annular plastic magnet and an outer diameter of the annular support member is set to be not less than 0.3 mm a gate of the injection molding mold being a disk gate located on an inner diameter side of the annular plastic magnet, the disk gate is located outwardly relative to a curved portion between the cylindrical portion and the flange portion of the annular support member, a length L of the disk gate in an axial direction being in a range of 0.2 mm ≤ L ≤ 0.6 mm, mechanical characteristics of a material of the annular plastic magnet being set such that a tensile strength is 65 MPa or more, and a Young's modulus is 4000 MPa or more and 15000 MPa or less, the method comprising: opening the injection molding mold and placing the annular support member as an insert object in the injection molding mold; and closing the injection molding mold and injecting the material of the annular plastic magnet that is melted, into a cavity in the injection molding mold through the disk gate, to perform injection molding of the annular plastic magnet having the shape. opening the injection molding mold and placing the annular support member as an insert object in the injection molding mold; and closing the injection molding mold and injecting the material of the annular plastic magnet that is melted, into a cavity in the injection molding mold through the disk gate, to perform injection molding of the annular plastic magnet having the shape.