Conventionally, a traveling vehicle etc.  which include a hydraulic stepless transmission (HST) which can change rotation speed and rotation direction of an output axis by way of a rotation operation of a trunnion axis have been publicly known.  With respect to such traveling vehicle including the hydraulic stepless transmission, a rotation position of the trunnion axis is detected by way of detecting operation amount of a speed change pedal which is an operation tool for operating the trunnion axis.  Specifically, for example, a rotation support shaft of the speed change pedal is provided with a potentiometer.  The potentiometer is configured to increase/decrease an output value at a constant rate proportionately with a rotation amount.  For example, such configuration can be found in Patent Literature 1. 
However, with regard to the technique disclosed in Patent Literature 1, when an operation speed of the speed change pedal is slow, increase/decrease of the output of the potentiometer per unit time also becomes slow.  However, when the output of the potentiometer per unit angle is amplified, the output value can be easily affected by a disturbance such as noise etc.  Particularly, on detecting the predetermined rotation position, the potentiometer can have a problem of a delay in detection or a detection error depending on the operation speed of the speed change pedal or a setting of the potentiometer.