1. A speech encoding apparatus for dividing an input speech into spectrum envelope information and excitation signal information and for encoding said excitation signal information by the frame, said speech encoding apparatus comprising: target speech generation means for generating from said input speech a target speech vector of a vector length corresponding to a delay parameter; an adaptive codebook for generating from previously generated excitation signals an adaptive vector of said vector length corresponding to said delay parameter; adaptive code search means for evaluating the distortion of a synthesis vector obtained from said adaptive vector with respect to said target speech vector so as to search for an adaptive vector conducive to the least distortion; and frame excitation generation means for generating an excitation signal of a frame length from said adaptive vector conducive to the least distortion, wherein said vector length of said target speech vector and said vector length of said adaptive vector are less than said frame length.