PURPOSE: To simplify adjustment of the gap between a rotary sensor and a pin for sensor, by incorporating the rotary sensor into the base stand as an idler part after fitting the rotary sensor to the idler arm.

CONSTITUTION: Before incorporating idler arm 27 into base stand 12, rotary sensor 31 and pin 30 for sensor are fitted to arm 27 to perform adjustment of the gap between sensor 31 and pin 30. After that, sensor 31 and pin 30 are made into an idler unit together with arm 27 and idler pulley 28, and this unit is incorporated into base stand 12 under a simple substance state. As a result, adjustment of the gap between the rotary sensor and the pin for sensor can be simplified.