A packet generation unit (10) generates a predetermined number of measurement packets at predetermined transmission intervals that increase in equal increments, and a transmission/reception unit (13) transmits the measurement packets to a reception device (2) in accordance with control performed by a transmission control unit (12). A calculation unit (21) calculates an available bandwidth in accordance with the reception intervals of the measurement packets received by a transmission/reception unit (20), and the transmission/reception unit (20) transmits the available bandwidth to a transmission device (1). A resolution alteration unit (14) determines, in accordance with the available bandwidth received by the transmission/reception unit (13), whether or not to change the measurement resolution. A size alteration unit (15) changes the minimum packet size, maximum packet size and packet size incremental increase amount when the resolution alteration unit (14) determines that the measurement resolution is to be changed. The packet generation unit (10) newly generates measurement packets in accordance with the changed values.