1. A speech coding apparatus comprising: an excitation searcher that performs an adaptive excitation search and fixed excitation search; an adaptive codebook that stores an adaptive excitation and clips a part of the adaptive excitation; a filter that performs predetermined filtering processing on the adaptive excitation clipped from the adaptive codebook; a fixed codebook that stores a plurality of fixed excitations and extracts a fixed excitation indicated by the excitation searcher; and a switch that switches between the adaptive excitation clipped by the adaptive codebook and the adaptive excitation after the filter processing by the filter, wherein the excitation searcher performs a search using the adaptive excitation clipped from the adaptive codebook, by controlling the switch to output the adaptive excitation clipped by the adaptive codebook during the adaptive excitation search, and performs a search using the adaptive excitation after the predetermined filtering processing, by controlling the switch to output the adaptive excitation after the filter processing by the filter during the fixed excitation search.