A controller of a cell phone serving as the position detector reads time-sequentially anteroposterior positional information Pi−1, Pi+1, together with interest positional information Pi that is an object of determination (S103), and calculates, for example, vectors V1 and V2 showing a shift from Pi−1, to Pi and a shift from Pi to Pi+1, respectively (S104, S105), in order to calculate an amplitude of fluctuation representing a direction of change. The amplitude of fluctuation representing a direction of change is expressed by a value of dot product of the calculated vectors V1 and V2 (S106). When the value of dot product is lower than a predetermined value (S107: YES), a position is shown to greatly deviate in the vicinity of the interest positional information Pi. Therefore, the amplitude of fluctuation representing a direction of change is judged to be large, and the positional information is determined to be error (S108).