A communication-line-quality estimating apparatus of a receiver adapted to receive at least two signals sent from a transmitter whose phase relation is known, the communication-line-quality estimating apparatus comprising: a first power-value calculating unit adapted to cumulatively add up and average a power value of a noise component calculated from a specified combination of the signals; a second power-value calculating unit adapted to cumulatively add up and average a power value of the signals; and a power-ratio calculating unit adapted to estimate communication line quality using a noise signal power value input from the first power-value calculating unit and a reception signal power value input from the second power-value calculating unit, characterized in that the communication-line-quality estimating apparatus further comprises: a threshold determining unit adapted to compare a communication line quality estimation result of the power-ratio calculating unit with a determination threshold, adapted to determine that, when the communication line quality estimation result is larger than the determination threshold, the communication line quality is satisfactory and to end the estimation of the communication line quality, and adapted to determine that, when the communication line quality estimation result is smaller than the determination threshold, the communication line quality is poor and to continue the estimation of the communication line quality; and a control unit adapted to control, on the basis of the determination of the threshold determining unit, whether signals of second and subsequent combinations are to be output to the first power-value calculating unit and whether the received signal is to be output to the second power-value calculating unit. a first power-value calculating unit adapted to cumulatively add up and average a power value of a noise component calculated from a specified combination of the signals; a second power-value calculating unit adapted to cumulatively add up and average a power value of the signals; and a power-ratio calculating unit adapted to estimate communication line quality using a noise signal power value input from the first power-value calculating unit and a reception signal power value input from the second power-value calculating unit, characterized in that the communication-line-quality estimating apparatus further comprises: a threshold determining unit adapted to compare a communication line quality estimation result of the power-ratio calculating unit with a determination threshold, adapted to determine that, when the communication line quality estimation result is larger than the determination threshold, the communication line quality is satisfactory and to end the estimation of the communication line quality, and adapted to determine that, when the communication line quality estimation result is smaller than the determination threshold, the communication line quality is poor and to continue the estimation of the communication line quality; and a threshold determining unit adapted to compare a communication line quality estimation result of the power-ratio calculating unit with a determination threshold, adapted to determine that, when the communication line quality estimation result is larger than the determination threshold, the communication line quality is satisfactory and to end the estimation of the communication line quality, and adapted to determine that, when the communication line quality estimation result is smaller than the determination threshold, the communication line quality is poor and to continue the estimation of the communication line quality; and a control unit adapted to control, on the basis of the determination of the threshold determining unit, whether signals of second and subsequent combinations are to be output to the first power-value calculating unit and whether the received signal is to be output to the second power-value calculating unit.