An engine comprising: a cylinder head; an exhaust valve housed inside the cylinder head; a valve mechanism for opening and closing the exhaust valve; a cam shaft for driving the valve mechanism by coming into contact with the valve mechanism; a bearing supporting the cam shaft in a rotatable manner on the cylinder head; and a decompression mechanism disposed between both ends of the cam shaft in an axial direction of the cam shaft; wherein the decompression mechanism includes: a weight supported on the cam shaft in a rotatable manner between a closed state and an open state; a return spring for urging the weight to return from the open state to the closed state; and a decompression cam provided so as to come into contact with the valve mechanism when the weight is in the closed state and so as not to come into contact with the valve mechanism when the weight is in the open state; when, as seen from the axial direction of the cam shaft, a straight line that passes through a center of rotation of the cam shaft and through a center of rotation of the weight is assumed to be a vertical axis, and a straight line that is orthogonal to the vertical axis and that passes through the center of rotation of the cam shaft is assumed to be a horizontal axis, and when a direction from the center of rotation of the cam shaft toward the center of rotation of the weight among directions parallel to the vertical axis is assumed to be a first vertical direction, and a direction parallel to the horizontal axis is assumed to be a first horizontal direction, the center of gravity of the weight is disposed in a first region that is located in the first vertical direction from the horizontal axis and in the first horizontal direction from the vertical axis as seen from the axial direction of the cam shaft; the weight includes a first portion that is disposed in the first region as seen from the axial direction of the cam shaft; and in a closed state, the first portion includes a first protruding portion that protrudes to the outside of an external peripheral surface of the bearing as seen from the axial direction of the cam shaft; and when a direction opposite the first vertical direction is assumed to be a second vertical direction, the weight further includes a second portion; and the second portion is disposed in a second region that is located in the second vertical direction from the horizontal axis and in the first horizontal direction from the vertical axis as seen from the axial direction of the cam shaft, characterized in that a maximum thickness of the first protruding portion in the axial direction of the cam shaft is greater than a maximum thickness of the second portion in the axial direction of the cam shaft. a cylinder head; an exhaust valve housed inside the cylinder head; a valve mechanism for opening and closing the exhaust valve; a cam shaft for driving the valve mechanism by coming into contact with the valve mechanism; a bearing supporting the cam shaft in a rotatable manner on the cylinder head; and a decompression mechanism disposed between both ends of the cam shaft in an axial direction of the cam shaft; a weight supported on the cam shaft in a rotatable manner between a closed state and an open state; a return spring for urging the weight to return from the open state to the closed state; and a decompression cam provided so as to come into contact with the valve mechanism when the weight is in the closed state and so as not to come into contact with the valve mechanism when the weight is in the open state; when, as seen from the axial direction of the cam shaft, a straight line that passes through a center of rotation of the cam shaft and through a center of rotation of the weight is assumed to be a vertical axis, and a straight line that is orthogonal to the vertical axis and that passes through the center of rotation of the cam shaft is assumed to be a horizontal axis, and when a direction from the center of rotation of the cam shaft toward the center of rotation of the weight among directions parallel to the vertical axis is assumed to be a first vertical direction, and a direction parallel to the horizontal axis is assumed to be a first horizontal direction, the center of gravity of the weight is disposed in a first region that is located in the first vertical direction from the horizontal axis and in the first horizontal direction from the vertical axis as seen from the axial direction of the cam shaft; the weight includes a first portion that is disposed in the first region as seen from the axial direction of the cam shaft; and in a closed state, the first portion includes a first protruding portion that protrudes to the outside of an external peripheral surface of the bearing as seen from the axial direction of the cam shaft; and when a direction opposite the first vertical direction is assumed to be a second vertical direction, the weight further includes a second portion; and the second portion is disposed in a second region that is located in the second vertical direction from the horizontal axis and in the first horizontal direction from the vertical axis as seen from the axial direction of the cam shaft,