1. A speech encoding apparatus for performing a search of an adaptive codebook of an enhancement layer for each sub-frame in scalable CELP encoding of a speech signal, the speech encoding apparatus comprising: a core layer encoding section that generates, for a core layer, a core layer excitation signal, and core layer encoded data that indicates an encoding result of CELP encoding, from the speech signal; an enhancement layer extended adaptive codebook generating section that generates, for the enhancement layer, an extended adaptive codebook that includes an enhancement layer excitation signal preceding in time a sub-frame targeted for encoding, and a core layer excitation signal succeeding in time past enhancement layer excitation signals; and an enhancement layer extended adaptive codebook that generates an enhancement layer adaptive excitation code indicating an adaptive excitation vector for the sub-frame targeted for encoding by searching in the generated extended adaptive codebook.