A stroke sensor is arranged to be offset downward relative to a pedal rotational axis, and link arms which interlock the stroke sensor with a pedal are provided. The link arm includes an engaged portion (groove), with which an engaging portion provided on the side of the pedal is slidably engaged. Herein, the engaged portion (groove) is configured such that the engaging portion is slid vertically along the engaged portion (groove) by a specified length that is long enough to allow falling-off of the pedal.