A valve train mechanism of an internal combustion engine which includes a cylinder head formed with an intake port and an exhaust port, and a camshaft (28, 33), the valve train mechanism comprising: a valve (20, 21) for opening or closing the intake port and the exhaust port formed in the cylinder head of the engine; a cam (29, 34) mounted to the camshaft so as to be rotated together; and a rocker arm (31, 36) supported on a rocker shaft (30, 35) to be swingable, wherein the valve is opened or closed by swinging the rocker arm by rotation of the cam, the rocker shaft is formed with an oil passage, inside the rocker shaft, extending in an axial direction and with an oil outlet communicating with the oil passage and extending in a radial direction of the rocker shaft, the rocker arm includes a bearing portion supported on the rocker shaft, the bearing portion being formed with an oil injection hole for injecting lubricating oil from the oil outlet to a sliding contact portion side between the rocker arm and the cam, and the oil injection hole and the oil outlet are constructed at least so as not to be overlapped with each other at a time when the rocker arm is in sliding contact with a base circle portion of the cam and so as to be overlapped with each other at a time when the rocker arm is in sliding contact with a nose portion of the cam in a predetermined press-down state, wherein as viewed from an axial direction of the camshaft, the rocker shaft is arranged on the upstream side in the rotational direction of the cam with respect to the sliding contact portion between the rocker arm and the cam, and that the rocker arm is configured to be rotatably supported at one of its longitudinal ends by the rocker shaft and to be pinched between the cam surface (29A, 34A) of the cam and an upper end of a valve stem of the valve, and wherein the valve includes an intake valve for opening or closing the intake port and an exhaust valve for opening or closing the exhaust port, and a valve angle centerline between the intake valve and the exhaust valve is set in a state biased, as viewed from the axial direction of the camshaft, toward an exhaust side than a cylinder axial line of the cylinder head. a valve (20, 21) for opening or closing the intake port and the exhaust port formed in the cylinder head of the engine; a cam (29, 34) mounted to the camshaft so as to be rotated together; and a rocker arm (31, 36) supported on a rocker shaft (30, 35) to be swingable, wherein the valve is opened or closed by swinging the rocker arm by rotation of the cam, the rocker shaft is formed with an oil passage, inside the rocker shaft, extending in an axial direction and with an oil outlet communicating with the oil passage and extending in a radial direction of the rocker shaft, the rocker arm includes a bearing portion supported on the rocker shaft, the bearing portion being formed with an oil injection hole for injecting lubricating oil from the oil outlet to a sliding contact portion side between the rocker arm and the cam, and the oil injection hole and the oil outlet are constructed at least so as not to be overlapped with each other at a time when the rocker arm is in sliding contact with a base circle portion of the cam and so as to be overlapped with each other at a time when the rocker arm is in sliding contact with a nose portion of the cam in a predetermined press-down state, wherein as viewed from an axial direction of the camshaft, the rocker shaft is arranged on the upstream side in the rotational direction of the cam with respect to the sliding contact portion between the rocker arm and the cam, and that the rocker arm is configured to be rotatably supported at one of its longitudinal ends by the rocker shaft and to be pinched between the cam surface (29A, 34A) of the cam and an upper end of a valve stem of the valve, and wherein the valve includes an intake valve for opening or closing the intake port and an exhaust valve for opening or closing the exhaust port, and a valve angle centerline between the intake valve and the exhaust valve is set in a state biased, as viewed from the axial direction of the camshaft, toward an exhaust side than a cylinder axial line of the cylinder head.