An internal combustion engine comprising: a cylinder head formed with a hole; 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 columnar support member, wherein at least a portion of the columnar support member is inserted into the hole of the cylinder head; a rocker arm that includes a supported portion pivotally supported on the support member, a pressed portion configured to be pressed by the cam (21A, 23A), and an abutting portion that is configured to abut on the valve (20, 22); and a securing member that includes a first contact portion (39B, 39C, 339a) that is configured to contact the support member, a second contact portion (39C, 39B, 339b) that is configured to contact the cylinder head, and an elastic portion (39A, 339c) interposed between the first contact portion (39A, 39C, 339a) and the second contact portion (39C, 39B, 339b), wherein the securing member is configured to secure the support member inside the hole, wherein the securing member is a plunger mechanism that includes a spring arranged inside the support member, and a presser, wherein at least a portion of the presser is arranged outside the support member and is connected to the spring, characterized in that a groove is formed on an inner surface of the hole of the cylinder head, wherein the groove is configured to engage with the securing member, wherein in a cross-section that passes through a part of the groove and that includes a center line of the hole, the groove has a sloped surface that is inclined relative to the center line of the hole so as to come closer to the center line of the hole while extending toward the rocker arm along a direction of the center line of the hole. a cylinder head formed with a hole; 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 columnar support member, wherein at least a portion of the columnar support member is inserted into the hole of the cylinder head; a rocker arm that includes a supported portion pivotally supported on the support member, a pressed portion configured to be pressed by the cam (21A, 23A), and an abutting portion that is configured to abut on the valve (20, 22); and a securing member that includes a first contact portion (39B, 39C, 339a) that is configured to contact the support member, a second contact portion (39C, 39B, 339b) that is configured to contact the cylinder head, and an elastic portion (39A, 339c) interposed between the first contact portion (39A, 39C, 339a) and the second contact portion (39C, 39B, 339b), wherein the securing member is configured to secure the support member inside the hole, wherein the securing member is a plunger mechanism that includes a spring arranged inside the support member, and a presser, wherein at least a portion of the presser is arranged outside the support member and is connected to the spring, characterized in that a groove is formed on an inner surface of the hole of the cylinder head, wherein the groove is configured to engage with the securing member, wherein in a cross-section that passes through a part of the groove and that includes a center line of the hole, the groove has a sloped surface that is inclined relative to the center line of the hole so as to come closer to the center line of the hole while extending toward the rocker arm along a direction of the center line of the hole.