A vehicle seat recliner for reclining a seat back with respect to a seat base, comprising: an internal gear member configured to be fixed to a first one of a frame (2a, 6) of the seat base and a frame of the seat back as a first frame (7; 2a, 6), wherein the internal gear member includes internal gear teeth; an external gear member configured to be fixed to a second one of the frame (2a, 6) of the seat base and the frame of the seat back as a second frame (2a, 6; 7), wherein the external gear member includes external gear teeth, and wherein the external gear teeth are smaller in tooth number than the internal gear teeth of the internal gear member, and are in mesh with the internal gear teeth of the internal gear member; a drive member (11, 17) supported rotatably in a central portion of the internal gear member, and configured to be rotated to cause relative revolution and relative rotation between the internal gear member and the external gear member; a lock member disposed between the internal gear member and the external gear member, and configured to selectively lock and unlock relative rotation between the internal gear member and the external gear member; a biasing member configured to bias the lock member in a direction to lock relative rotation between the internal gear member and the external gear member; and a cover configured to cover an outside of the external gear member, and accommodate the biasing member between the cover and the external gear member; wherein the outside of the external gear member includes fitting projections, wherein the fitting projections project toward the cover, and are arranged concentrically; and each of the fitting projections includes a radial inside fixed to the cover by press-fitting and recess-projection fitting, and includes a radial outside configured to be fitted to a mounting hole of the second frame (2a, 6; 7). an internal gear member configured to be fixed to a first one of a frame (2a, 6) of the seat base and a frame of the seat back as a first frame (7; 2a, 6), wherein the internal gear member includes internal gear teeth; an external gear member configured to be fixed to a second one of the frame (2a, 6) of the seat base and the frame of the seat back as a second frame (2a, 6; 7), wherein the external gear member includes external gear teeth, and wherein the external gear teeth are smaller in tooth number than the internal gear teeth of the internal gear member, and are in mesh with the internal gear teeth of the internal gear member; a drive member (11, 17) supported rotatably in a central portion of the internal gear member, and configured to be rotated to cause relative revolution and relative rotation between the internal gear member and the external gear member; a lock member disposed between the internal gear member and the external gear member, and configured to selectively lock and unlock relative rotation between the internal gear member and the external gear member; a biasing member configured to bias the lock member in a direction to lock relative rotation between the internal gear member and the external gear member; and a cover configured to cover an outside of the external gear member, and accommodate the biasing member between the cover and the external gear member; wherein the outside of the external gear member includes fitting projections, wherein the fitting projections project toward the cover, and are arranged concentrically; and each of the fitting projections includes a radial inside fixed to the cover by press-fitting and recess-projection fitting, and includes a radial outside configured to be fitted to a mounting hole of the second frame (2a, 6; 7).