A transmission power control method includes a step (a) of receiving by a receiver a radio wave signal, a step (b) of judging whether a first identification signal is included in the received radio wave signal, a step (c) of detecting a reception electric field intensity of the radio wave signal included in the radio wave signal in the case where the first identification signal is included in the radio wave signal, a step (d) of obtaining a radio wave propagation state between a home station and the station to communicate with by using the reception electric field intensity detected at the step (c) and determining transmission power of the transmission radio wave signal based upon the radio wave propagation state, a step (g) of invalidating the radio wave propagation state obtained upon the previous reception in the case where the interval between an end of communication and a restart of communication is longer than a time determined in advance, a step (e) of superimposing a second identification signal on the transmission radio wave signal and a step (f) of controlling such that the transmission power of the transmission radio wave signal becomes the value determined at the step (d).