PURPOSE: To obtain a sample holding signal which is close to an original signal, by detecting the frequency and level of the original signal and then varying the time constant of the sample holding signal in response to said frequency and level of the original signal.

CONSTITUTION: The voice frequency part of an original signal supplied via a terminal 8 is filtered through a BPF3, and the voice frequency (f) is detected at a frequency detecting part 2. When a drop-out arises to the original signal, a drop-out detecting part 5 detects the drop-out time tDO. A drop-out comparing part 6 performs a comparing operation between the time tDO and 1/2f based on the frequency (f) and the time tDO. At the same time, the level of the original signal is detected at a level detecting part 4. Then the signal showing the result of the above-mentioned operation is supplied to a time constant varying part 7, and the part 7 delivers a signal which shows the sample holding time constant in response to the original signal.