A technique of improving the degree of freedom of controlling the accuracy of encoding a stereo signal. In a stereo signal encoding device, a sum/difference calculation section generates a monophonic signal which is the sum of first and second channel signals constituting a stereo signal and a side signal which is the difference between the first channel signal and the second channel signal. A mode setting section generates mode information that indicates either a monophonic encoding mode or a stereo encoding mode. A core layer encoding section, a first extended layer encoding section, a second extended layer encoding section, and a third extended layer encoding section individually carry out the monophonic encoding using the monophonic signals or the stereo encoding using both the monophonic signal and the side signal depending on the mode information, and output to a multiplexing section the resultant encoded information from the core layer to the third extended layer.