A speech coding apparatus comprising: an adaptive codebook that outputs an adaptive codebook vector representing a periodic component; a fixed codebook that outputs a fixed codebook vector representing a non-periodic component; an adder that generates an excitation signal from the adaptive codebook vector and the fixed codebook vector; a synthesis filter that operates based on a linear prediction coefficient obtained by performing linear prediction analysis on an input speech signal and quantization and that is driven by the excitation signal thereby generating a synthesized speech signal; and a parameter quantization unit that selects the adaptive codebook vector and the fixed codebook vector so as to minimize an error between the synthesized speech signal and the input speech signal, characterized in that the parameter quantization unit includes a fixed codebook search unit that switches between an orthogonal fixed codebook search and a non-orthogonal fixed codebook search based on a correlation value between a target vector for the fixed codebook search and the adaptive codebook vector obtained as a result of the process by the synthesis filter, the process by the synthesis filter includes a process for convoluting the adaptive codebook vector with an impulse response of the synthesis filter or a process for convoluting the adaptive codebook vector with an impulse response of both the synthesis filter and a perceptual weighting filter, and/or the parameter quantization unit includes a fixed codebook search unit that switches between an orthogonal fixed codebook search and a non-orthogonal fixed codebook search based on a distance between a first vector product matrix of a target vector for the adaptive codebook search and the adaptive codebook vector obtained as a result of the synthesis filtering process and a second vector product matrix of the adaptive codebook vector obtained as the result of the synthesis filtering process and the adaptive codebook vector obtained as a result of the synthesis filtering process. an adaptive codebook that outputs an adaptive codebook vector representing a periodic component; a fixed codebook that outputs a fixed codebook vector representing a non-periodic component; an adder that generates an excitation signal from the adaptive codebook vector and the fixed codebook vector; a synthesis filter that operates based on a linear prediction coefficient obtained by performing linear prediction analysis on an input speech signal and quantization and that is driven by the excitation signal thereby generating a synthesized speech signal; and a parameter quantization unit that selects the adaptive codebook vector and the fixed codebook vector so as to minimize an error between the synthesized speech signal and the input speech signal, characterized in that the parameter quantization unit includes a fixed codebook search unit that switches between an orthogonal fixed codebook search and a non-orthogonal fixed codebook search based on a correlation value between a target vector for the fixed codebook search and the adaptive codebook vector obtained as a result of the process by the synthesis filter, the process by the synthesis filter includes a process for convoluting the adaptive codebook vector with an impulse response of the synthesis filter or a process for convoluting the adaptive codebook vector with an impulse response of both the synthesis filter and a perceptual weighting filter, and/or the parameter quantization unit includes a fixed codebook search unit that switches between an orthogonal fixed codebook search and a non-orthogonal fixed codebook search based on a distance between a first vector product matrix of a target vector for the adaptive codebook search and the adaptive codebook vector obtained as a result of the synthesis filtering process and a second vector product matrix of the adaptive codebook vector obtained as the result of the synthesis filtering process and the adaptive codebook vector obtained as a result of the synthesis filtering process.