PURPOSE: To attain the discrimination of the presence of carrier accurately even if an environmental noise level is large by enabling the transmission of a transmission section if either a carrier detection output of a level discrimination section or a signal output of a signal detection section is absent.

CONSTITUTION: A pilot signal detection section 7 extracts and detects a pilot signal P included in a subcarrier demodulated by a reception section 2. If the level of environmental noise is low, a level of a monitor signal S exceeds a discrimination level L, a discrimination output of carrier detection is generated from a level discrimination section 4 and in the presence of detection of a signal P, a transmission control section 8 stops the transmission and in other cases, the transmission is enabled. Moreover, when the environmental noise level is high, the level of the signal S exceeds the level L in the absence of carrier and a discrimination output is generated from the discrimination section 4, but in this case, since the detection output of the signal P is not generated, the transmission enable is attained and the transmission is suppressed after the signal P is detected.