A method for determining a periodic feature amount of an audio signal in frames, the method comprising: a periodic feature amount determination step of determining a periodic feature amount of the audio signal from a set of candidates for the periodic feature amount on a frame-by-frame basis; and a side information generating step of encoding the periodic feature amount obtained at the periodic feature amount determination step to obtain side information; wherein the periodic feature amount determination step determines a periodic feature amount from a set S of candidates for the periodic feature amount, the set S being made up of Y candidates among Z candidates for the periodic feature amount, the Y candidates including Z 2 candidates selected without depending on a candidate subjected to the periodic feature amount determination step in a previous frame a predetermined number of frames before the current frame and being able to include one or more candidates subjected to the periodic feature amount determination step in the previous frame the predetermined number of frames before the current frame, the Z candidates being representable with the side information, where Z 2 < Z and Y < Z, characterized in that: the greater an indicator indicating the degree of stationarity of the audio signal in the current frame, the greater the proportion of candidates subjected to the periodic feature determination step in the previous frame the predetermined number of frames before the current frame to the set S is. a periodic feature amount determination step of determining a periodic feature amount of the audio signal from a set of candidates for the periodic feature amount on a frame-by-frame basis; and a side information generating step of encoding the periodic feature amount obtained at the periodic feature amount determination step to obtain side information; wherein the periodic feature amount determination step determines a periodic feature amount from a set S of candidates for the periodic feature amount, the set S being made up of Y candidates among Z candidates for the periodic feature amount, the Y candidates including Z 2 candidates selected without depending on a candidate subjected to the periodic feature amount determination step in a previous frame a predetermined number of frames before the current frame and being able to include one or more candidates subjected to the periodic feature amount determination step in the previous frame the predetermined number of frames before the current frame, the Z candidates being representable with the side information, where Z 2 < Z and Y < Z, characterized in that: the greater an indicator indicating the degree of stationarity of the audio signal in the current frame, the greater the proportion of candidates subjected to the periodic feature determination step in the previous frame the predetermined number of frames before the current frame to the set S is. the greater an indicator indicating the degree of stationarity of the audio signal in the current frame, the greater the proportion of candidates subjected to the periodic feature determination step in the previous frame the predetermined number of frames before the current frame to the set S is.