1. A speech coder that is a code excited linear prediction type speech coder, the speech coder comprising: a seed storage that stores a plurality of seeds used as an initial state of oscillation; an oscillator that generates different vector sequences in accordance with values of the seeds stored in the seed storage and outputs the vector sequences as excitation vectors; and a linear predictive coding synthesis filter that receives as input, the excitation vectors which are the vector sequences generated in accordance with the values of the seeds, synthesizes the excitation vectors, and outputs a synthesized speech, wherein: the seed storage stores the plurality of seeds prepared in advance as the initial state of oscillation such that the vector sequences generated in the oscillator serve as effective excitation vectors from which the synthesized speech can be generated when the vector sequences are input to the linear predictive coding synthesis filter; and the oscillator receives as input, the seeds from the seed storage, generates, using the input seeds, vector sequences that serve as the effective excitation vectors from which the synthesized speech can be generated in the linear predictive coding synthesis filter, and outputs the vector sequences.