PURPOSE: To improve the reliability of an electromagnetic pick-up sensor for detecting a fin part of a rotor member by supplying oil to a shift drum with a rotor member for detecting the rotation condition of a main shaft.

CONSTITUTION: A speed change gear mechanism is switched by a shift drum 80 through plural shift rods 71, 72, 73, and a rotor member 96 for detecting the rotation condition of a main shaft 45 is provided in the main shaft 45 near the shift drum 80. Three electromagnetic pick-up sensors 101, 102, 103 for detecting a fin part 97 of the rotor member 96 are provided in a casing so that a detecting end thereof are arranged lower than the oil surface 98 of the oil. Oil is splashed by the rotor member 96 to be supplied to the shift drum 80, and the sensor detecting ends are always cleaned by the agitated flow of the oil to secure the reliability of the sensors 101, 102, 103.