PURPOSE: To adjust a detecting sensor on a position indicating maximum sensitivity by adjusting an eccentric type fixing member so that a sprocket pin can adjust the crossing position of light detected by the detecting sensor.

CONSTITUTION: When the eccentric fixing part 4A for eccentrically fixing a fixing member 4 from the axis (a) of a sprocket wheel 1 is rotated in an arrow D or D' direction to adjust the position of the detecting sensor 3 from the sprocket pin 1A, the position of detection light l of the sensor 3 from the pin 1A is moved in the vertical direction, i.e. the radius direction of the wheel 1, and a position crossing the detection light l of the pin 1A is changed between l1 l2. Thereby when the detection light l from a light emitting part 3A is received by a light receiving part 3B, the fixing part 4A can be fixed and held on a position from which the maximum detection output (sensitivity) can be obtained by means for a fixing screw 5. Thus, the detection sensor can be adjusted on the position having proper sensitivity.