This paper proposes a high speed bit rate reduction technique on the compressed domain for AAC audio. There are many kinds of AAC audio devices such as PCs and mobile terminals. In order to adapt to various requirements on audio quality and storage capacity, a fast audio transcoding method from a higher to a lower bit rate would be beneficial for the efficient use of master AAC contents. Although the scalability function of AAC or baseband transcoding, which converts a bitstream by decoding and re-encoding, can be employed for this purpose, lower coding efficiency or higher computation load is induced. To resolve these problems, this paper derives the relationship among MDCT, quantization and scaling, and achieves a fast and high quality bit rate reduction by introducing (1) an adaptive scalefactor control, (2) MDCT gain control and 3) the reuse of side information. Experimental results show that the proposed method is about 1.4 times faster than baseband transcoding while achieving almost the equivalent quality.