in communication with the return port 10d as seen in fig1 the control valve 7 is mounted to the cylinder head inclined relative to a horizontal line parallel to the crankshaft of the engine preferably inclined relative to that line by about 25 degrees in the embodiment shown in fig1 the control valve is mounted at an upper portion of the cylinder head relatively higher than other portions of the head when the motorcycle is supported by the kickstand 26 to thereby incline the cylinder head slightly namely the control valve 7 is mounted at an upper portion of the cylinder head at a position which corresponds to an upper portion of a line extending perpendicular to the crankshaft that is a line extending transversally in fig1 when the kickstand is in use in other words the control valve 7 is mounted at the upper portion at one side of the cylinder head which is opposite the side of the kickstand the control valve 7 is mounted to the cylinder head in such a manner that an oil ejecting opening 31 where the outlet port 10c debouches into the oil path 5d is disposed slightly higher than the pipe 14 when the kickstand is in use as shown in fig1 in the energized state of the control valve the oil ejecting opening 31 always is at a position slightly higher than the pipe 14 not only when the motorcycle is upright but also when the motorcycle is supported by the kickstand accordingly.