Disclosed is a stereo acoustic signal encoding apparatus in which the signal quality does not deteriorate if there are a plurality of sound sources. A peak tracing unit (401) splits frames of a right channel signal and a left channel signal into a plurality of sub frames; detects the peaks of wave shapes of the split sub frames; and estimates a frame delay time D for each frame of the right channel signal and the left channel signal by comparing the positions of the detected peaks. A time adjusting unit (402) adjusts the time of the right channel signal on the basis of the frame time delay D. A down-mix operation is carried out using the right channel signal which has been subjected to the time adjustment and the left channel signal to generate a mono signal and a sub signal. A mono signal encoding unit (403) encodes the mono signal. A sub signal encoding unit (404) encodes the sub signal. The time delay encoding unit (405) encodes the frame time delay D.