An amplifier device includes a plurality of input terminals for receiving input signals (e.g. audio signals), a main switch, which switches over the input terminals so as to select one input terminal, an analyzer, which analyzes the input signals at respective timings so as to determine whether or not each of the input signals corresponds to a playback signal, a storage for storing a plurality of determination results in connection with the input terminals, a controller for controlling the main switch to select the input terminal whose input signal is newly determined as the playback signal, and an amplifier for amplifying the input signal of the selected input terminal. The analyzer performs frequency analysis on the input signals so as to determine the input signal including a specific frequency component having a high intensity as noise not forming the playback signal.