1. A speech coding apparatus for coding an input speech on a frame-by-frame basis using an adaptive excitation source, which is generated from a past excitation source, and a driving excitation source, which is generated from the input speech and the adaptive excitation source, so as to generate speech code, said speech coding apparatus comprising: a repetition period pre-selecting means for generating a plurality of candidates for a repetition period of the driving excitation source by multiplying a repetition period of the adaptive excitation source by a plurality of constant numbers, respectively, and for pre-selecting a predetermined number of candidates from all the candidates generated and furnishing the predetermined number of pre-selected candidates; a driving excitation source coding means for providing both excitation source location information and excitation source polarity information that minimize a coding distortion, for each of the predetermined number of candidates for the repetition period of the driving excitation source, and for providing an evaluation value associated with the minimum coding distortion for each of the predetermined number of candidates; and a repetition period coding means for comparing the evaluation values provided for the predetermined number of candidates for the repetition period of the driving excitation source from said driving excitation source coding means with one another, for selecting one candidate from the predetermined number of candidates according to a comparison result, and for furnishing selection information indicating a selection result, excitation source location code indicating excitation source location information associated with the selected candidate for the repetition period of the driving excitation source, and polarity code indicating excitation source polarity information associated with the selected candidate.