A level of an input signal is detected according to a first following rate, and a first level signal indicating the detected level is generated. A level of the input signal is detected according to a second following rate lower than the first following rate, and a second level signal indicating the detected level is generated. One of the first and second level signals is selected based on a relation (e.g., ratio) between the first and second level signals so that a gain is determined based on the selected one of the first and second level signals. The level of the first input signal is adjusted according to the determined gain. For example, if the level variation is dominant, the gain adjustment suitable for the level variation can be performed, whereas, if the stable level is dominant, the gain adjustment suitable for the stable level can be performed.