When viewed from the upper face of a cylinder head (52A) in which an intake port (10A) communicating with a combustion chamber (54) of an internal combustion engine (50A) is formed, a starting point (Pl) of the port (10A) is defined as a point of intersection of the streamline of the intake port (10A) and an inlet-side opening plane of the intake port (10A) as projected on a horizontal plane (S), and an end point (P2) is defined as a point of intersection of the streamline of the port (10A) and the center -axis of an intake valve (55) as projected on the horizontal plane (S). The starting point (Pl) is located closer to the center of the chamber (54) than a straight line (L2) that contains the end point (P2) and extends in a direction orthogonal to the axis of the crankshaft on the plane (S), and the streamline (F) of the port (10A) projected on the horizontal plane (S) is curved toward the center of the chamber (54), with respect to a straight line (L1) that contains the starting point (P1) and extends in a direction orthogonal to the axis of the crankshaft on the plane (S).