A communication-quality measuring method disclosed in the present invention acquires packets to be transferred in a packet switching network and accumulates the acquired packets in an internal buffer. An acquisition amount indicating an amount of packets acquired per given unit time, a loss amount indicating an amount of packets lost during transfer in the packet switching network, and an acquisition leak amount indicating an amount of packets that cannot be accumulated in the internal buffer out of the acquired packets are measured. Based on the measured acquisition amount, loss amount, and acquisition leak amount, a packet loss rate transferred in the packet switching network and an error for the loss rate are calculated and the calculated loss rate and the loss rate error are output.