An audio encoding apparatus and an audio encoding method, wherein the processing amount can be reduced and a block length can be appropriately selected. An electric power calculation part (402) calculates, from an input signal, an electric power variation ratio. A predicted gain variation ratio calculation part (406) calculates, from the input signal, a predicted gain variation ratio. A block length determination part (407) determines, from the electric power variation ratio and the predicted gain variation ratio, whether an encoding using a long block is to be executed or an encoding using a short block is to be executed. Based on this determination, a long block MDCT conversion part (409) or a short block MDCT conversion part (410) performs a discrete cosine transformation of the input signal.