A device includes: a storage that stores detection values of a position detector when an operation member is at a start point position and when the operation member is at an end point position as a start point detection value and an end point detection value, respectively; an operation amount computing section that, based on a difference between the detection value of the position detector and the start point detection value, computes an operation amount of the operation member; and a correction section that, when the detection value falls below the start point detection value, corrects the start point detection value by adding thereto a deviation value of the detection value from the start point detection value, and when the detection value exceeds the end point detection value, corrects the start point detection value by adding thereto a deviation of the detection value from the end point detection value.