From an antenna provided in a transmitter for transmitting a radio signal, the radio signal is transmitted in such a manner of: detecting transmission power of a transmission signal outputted to the antenna; detecting reflection power of a reflection signal reflected from the antenna; integrating the difference between the transmission power and the reflection power at a timing of transmitting a preamble signal of the radio signal; comparing the difference value obtained by the integration with a predetermined threshold value, and outputting an alarm in the case where the result of the comparison is that the difference value obtained by the integration is smaller than the threshold value.