PURPOSE: To ensure the level control of other SCPC wave amplified in common and to improve the reliability by selecting a preset standby SCPC wave to continue the automatic gain control operation when a fault takes place in the level of the SCPC wave subjected to automatic gain control.

CONSTITUTION: A level detector comprises a frequency converter 1, a band pass filter 2, a detector 3, an A/D converter 4, a logic controller 5, and a frequency synthesizer 6 are provided with a storage device 7, which stores the level of a standby SCPC(Single Channel Per Carrier) wave applying automatic gain control and outputting a stored value to the logic controller 5 when a fault takes place in the SCPC wave. Thus, when a fault takes place in the SCPC wave subjected to automatic gain control, the standby SCPC wave whose level is stored in the storage device 7 is used to keep the level detection, thereby ensuring the level control in the other SCPC wave.