A sound encoder having an improved quantization performance while suppressing an increase of the bit rate to a lowest level. In a second layer encoder, a standard deviation calculator calculates a standard deviation σc of a first layer decoding spectrum after decoding a scale factor ratio multiplication and outputs the standard deviation σc to a selector. The selector selects a linear transform function as a function for a nonlinear transform of a residual spectrum according to the standard deviation σc A nonlinear transform function selects one of prepared nonlinear transform functions #1 to #N according to a result of the selection by the selector, and outputs the selected one to an inverse transformer. The inverse transformer subjects an inverse transform (expansion) to a residual spectrum candidate that is stored in a residual spectrum code book using the nonlinear transform function outputted from the nonlinear transform function and outputs the result to an adder.