PURPOSE: To realize memorization of the microwave without disturbing the spectrum by giving the minor signal suppression properties to the loop amplifier or providing the minor signal suppression circuit which ensures an overall equivalent action.

CONSTITUTION: Minor signal suppression circuit 9 is added to loop amplifier 3 to secure the minor signal suppression properties. Circuit 9 consists of hybrid 10 of dB90°, limiter 11 and resistive terminator 12. In case the input signal level is lower than the limiting level of limiter 11, the input signal is applied to terminator 12 and not delivered to terminal 10-2 of hydrid 10. While in case the input signal level is higher than the limiting level, the reflection coefficient of limiter 11 increases. As a result, the input signal is reflected by terminals 10-3 and 10-4 of hybrid 10 to be delivered to terminal 10-2. In such way, the minor signal can be suppressed centering on the limiting level of limiter 11.