A stereo sound decoding apparatus wherein lost-frame compensation performance has been improved to enhance the quality of decoded sounds. In this stereo sound decoding apparatus, a sound decoding part uses encoded monophonic signal data and encoded side signal data, which are received from a sound encoding apparatus, to generate monophonic decoded signals and stereo decoded signals; a compensation signal switching determining part that compares an inter-channel correlation and an intra-channel correlation, which have been calculated by use of the monophonic decoded signals of a previous frame and the stereo decoded signals of the previous frame, with respective comparison thresholds; a compensation signal switching part that selects, based on a result of the comparison in the compensation signal switching determining part, as compensation signals either inter-channel compensation signals generated by an inter-channel compensating part or intra-channel compensation signals generated by an intra-channel compensating part; and an output signal switching part that outputs either the stereo decoded signals or the compensation signals according to whether the encoded side signal data of the current frame has been lost.