The risk of a third party able to conduct pairing is reduced, and operation is also made simple. In S1 the radio wave strength of a pairing signal received first is measured and retained. In S2 the message “Come a little closer” is displayed. In S4 it is determined from the radio wave strength whether or not the current distance is within a given range with respect to an instructed distance. In S5 it is determined whether or not the number of determinations in which the current distance is determined to be within the given range has reached a set number of determinations. In a second determination the message “Now move away a little” is displayed. If the second determination becomes positive, S6 (pairing registration) is performed.