1. A speech decoding method for decoding a speech code including a linear prediction parameter code, an adaptive code, and a gain code according to code-excited linear prediction (CELP), the speech decoding method comprising: obtaining an adaptive code vector from an adaptive codebook based on the adaptive code; obtaining a time series vector from an excitation codebook based on the excitation code; decoding a gain of the adaptive code vector and a gain of an excitation code vector from the gain code; classifying the gain of the adaptive code vector into a first gain corresponding to a first noise level or a second gain corresponding to a second noise level, wherein the first noise level is different from the second noise level; determining a value to be used in modifying the time series vector based on the results of classifying the gain of the adaptive code vector; performing a mathematical operation on the time series vector according to the determined value; weighting the adaptive code vector and the time series vector using the decoded gains as weights; obtaining an excitation signal by adding the weighted adaptive code vector and the weighted time series vector; decoding a linear prediction parameter from the linear prediction parameter code; and synthesizing a speech using the linear prediction parameter and the excitation signal.