a receiver configured to receive a radio signal;

a calculation unit configured to calculate a channel response by use of the radio signal;

a channel response variation measurement unit configured to measure a change in amplitude or phase from that of a previously calculated channel response every time a channel response is calculated by the calculation unit, to determine a variation of a radio propagation environment based on the change;

a setting unit configured to set a frequency of insertion of a plurality of preamble signals based on the variation, wherein the preamble signals are used for calculating a channel response at a receiving side;

a transmitted signal generator configured to generate a transmitted signal by inserting the preamble signals in accordance with the frequency of insertion, wherein the preamble signals are separated with respect to time; and

a control signal field generator configured to generate an output frequency field indicating the frequency of insertion of the preamble signals,

wherein said transmitted signal generator inserts the output frequency field into the transmitted signal, and

the setting unit refers to the output frequency field contained in the received signal received by the receiver so as to control the frequency of insertion based on the output frequency field.