A strain quantity obtaining unit is provided that obtains a strain quantity at an interval faster than variation in the strain to be detected and stores the maximum value of the strain quantity. When a touch coordinate is obtained at a predetermined sampling interval, if the maximum value of the strain quantity when the touch coordinate is obtained does not exceed a strain quantity threshold value, the touch coordinate is not caused to be effective. However, if the maximum value of the strain quantity when the touch coordinate is obtained exceeds a strain quantity threshold value, the touch coordinate is caused to be effective. If a touch coordinate is caused to be effective once, the touch coordinate is caused to be effective continuously until an instructing object is separated from an operation surface of a touch panel unit at a predetermined distance or more in a perpendicular direction.