1. A speech coding apparatus, comprising: a perceptual linear prediction (plp) analysis buffer configured to output a pitch period with respect to an original input speech signal and to analyze the input speech signal using a plp process to output a plp coefficient; an excitation signal generator configured to generate and output an excitation signal; a pitch synthesis filter configured to synthesize the pitch period output from the plp analysis buffer and the excitation signal output from the excitation signal generator; a spectral envelop filter configured to apply the plp coefficient output from the plp analysis buffer to an output of the pitch synthesis filter so as to output a synthesized speech signal; an adder configured to subtract the synthesized signal output from the spectral envelope filter from the original input speech signal output from the plp analysis buffer and to output a difference signal; a perceptual weighting filter configured to calculate an error by providing a weight value corresponding to a consideration of a person's auditory effect to the difference signal output from the adder; and a minimum error calculator configured to discover an excitation signal having a minimum error corresponding to the error output from the perceptual weighting filter, wherein the excitation signal generator includes a codebook index and a codebook gain of a codebook, and said apparatus further comprises a searching unit configured to search the excitation signal having the minimum error from the codebook, the apparatus further comprising a transmitter configured to transmit the codebook index, the codebook gain, the pitch period and the plp coefficient to an intended user.