An internal combustion engine comprising: a cylinder head; a port (14, 16) formed in the cylinder head; a valve (20, 22) that is installed in the cylinder head and that is configured to open/close the port (14, 16); a cam shaft (21, 23) rotatably supported on the cylinder head; a cam (21A, 23A) provided on the cam shaft (21, 23); a rocker arm including a first arm, a second arm and a roller, wherein the first arm includes a supported portion pivotally supported on the cylinder head and a contact portion that is configured to contact the valve (20, 22), the second arm is pivotally supported on the first arm, and the roller is rotatably attached to the second arm and arranged between the supported portion and the contact portion of the first arm; a connecting mechanism that removably connects the first arm and the second arm; a cotter attached to the valve (20, 22); a valve spring retainer to which the cotter is fitted and through which the valve (20, 22) passes; and a coil spring that includes a first spring end portion supported on the flange portion of the valve spring retainer and a second spring end portion supported on the cylinder head, wherein the valve spring retainer comprises: a cylindrical portion having a first end portion and a second end portion, wherein the cylindrical portion is formed with a first through hole having an inner diameter decreasing from the first end portion toward the second end portion; a cone-shaped portion extending from the second end portion of the cylindrical portion along an axial direction of the cylindrical portion, wherein the cone-shaped portion is formed with a second through hole having an inner diameter increasing in a direction away from the second end portion; and a flange portion extending radially outward from the cone-shaped portion, wherein: an outer diameter of the cylindrical portion is constant from the first end portion to the second end portion; and an outer diameter of the cone-shaped portion increases in a direction away from the second end portion. a cylinder head; a port (14, 16) formed in the cylinder head; a valve (20, 22) that is installed in the cylinder head and that is configured to open/close the port (14, 16); a cam shaft (21, 23) rotatably supported on the cylinder head; a cam (21A, 23A) provided on the cam shaft (21, 23); a rocker arm including a first arm, a second arm and a roller, wherein the first arm includes a supported portion pivotally supported on the cylinder head and a contact portion that is configured to contact the valve (20, 22), the second arm is pivotally supported on the first arm, and the roller is rotatably attached to the second arm and arranged between the supported portion and the contact portion of the first arm; a connecting mechanism that removably connects the first arm and the second arm; a cotter attached to the valve (20, 22); a valve spring retainer to which the cotter is fitted and through which the valve (20, 22) passes; and a coil spring that includes a first spring end portion supported on the flange portion of the valve spring retainer and a second spring end portion supported on the cylinder head, wherein the valve spring retainer comprises: a cylindrical portion having a first end portion and a second end portion, wherein the cylindrical portion is formed with a first through hole having an inner diameter decreasing from the first end portion toward the second end portion; a cone-shaped portion extending from the second end portion of the cylindrical portion along an axial direction of the cylindrical portion, wherein the cone-shaped portion is formed with a second through hole having an inner diameter increasing in a direction away from the second end portion; and a flange portion extending radially outward from the cone-shaped portion, wherein: a cylindrical portion having a first end portion and a second end portion, wherein the cylindrical portion is formed with a first through hole having an inner diameter decreasing from the first end portion toward the second end portion; a cone-shaped portion extending from the second end portion of the cylindrical portion along an axial direction of the cylindrical portion, wherein the cone-shaped portion is formed with a second through hole having an inner diameter increasing in a direction away from the second end portion; and a flange portion extending radially outward from the cone-shaped portion, wherein: an outer diameter of the cylindrical portion is constant from the first end portion to the second end portion; and an outer diameter of the cone-shaped portion increases in a direction away from the second end portion.