The amount of computation in an acoustic signal decoding apparatus for a signal transform process from a frequency domain to a time domain is reduced while realizing the generation of appropriate output acoustic signals.An output control unit 340 receives, from a code string separating unit 310, pieces of window information including a window shape showing the type window function related to a windowing process of input channels, and, if all the pieces of window information are the same, switches the connections of output switching units 351 to 355 to a frequency domain mixing unit 510. The frequency domain mixing unit 510 mixes frequency domain signals of five channels supplied from a decoding/dequantizing unit 320 on the basis of downmix information that causes the number of output channels to be smaller than the number of input channels. IMDC/windowing processing units 521 and 522 transform frequency domain signals of two channels output from the frequency domain mixing unit 510 into time domain signals, thereby outputting the signals as acoustic signals of two channels.