A nonresponse-ratio determining unit determines, after starting a signal transmission to an electronic tag, whether a ratio of number of nonresponses from the electronic tag exceeded a predetermined value in a first predetermined period. A signal-transmission terminating unit terminates, when it is determined that the ratio of the number of nonresponses exceeded the predetermined value, the signal transmission for a second predetermined period. A radio-frequency-identification interrogator performs, after a lapse of the second predetermined period, a communication-channel selecting process for restarting the signal transmission to the electronic tag.