An RFID tag distance measuring system includes a reader that outputs a predetermined carrier signal to a tag and receives a reflected signal of the carrier signal from the tag. The reader has signal output means for outputting signals at multiple frequencies, which are different from each other, as the carrier signal, a transmitting section that transmits a signal output by the signal output means to the tag, a receiving section that receives reflected signals of the signals at multiple frequencies, which are different from each other, from the tag and estimating means for estimating the distance between the tag and the reader based on the amount of change in phase between the reflected signals received by the receiving section and the carrier signals and the frequencies of the carrier signals. The transmitting section and the receiving section have two pairs of transmit and receive antennas that obtain multiple receive signals in which the strengths of the reflected signals vary differently when the receiving section receives the reflected signals.