The present invention relates to an ambient field level monitor device for a radio mobile terminal, comprising a reception unit for receiving a radio signal arriving at the radio mobile terminal to output a reception signal originating from the received radio signal and further to output an ambient field level signal, a synchronization decision section for receiving the reception signal from the reception unit to judge whether or not the reception unit is in synchronism with the received radio signal, and a time constant variable type ambient field level monitoring unit for receiving the ambient field level signal from the reception unit. If the synchronization decision section judges that the reception unit is out of synchronism with the received radio signal, the ambient field level monitoring unit increases its time constant so that the time constant exceeds its time constant to be taken for when the synchronization decision section judges that the reception unit is in synchronism with the received radio signal and if the synchronization decision section judges that the reception unit is synchronism with the radio signal, the ambient field level monitoring unit decreases the time constant. This reduces the measurement error caused by the difference between sampling timings, thereby enabling stable RSSI value detection irrespective of a phase synchronization condition of a mobile station to improve the detection accuracy.