A multiplexed audio data decoder apparatus is provided in which integration of an audio decoder is easy, and has a high flexibility when the number of the formats to be processed is increased or when the specification is changed. In an external ROM 60 there are accumulated a plurality of decoding program codes corresponding to respective plural methods for compressing and encoding. A controller 50 transfers the decoding program code corresponding to the method for compressing and encoding after changing thereof, from the external ROM 60 to an internal RAM 25. A DSP 22 starts decoding processing by using the decoding program code which is transmitted into the internal RAM 25.