A speech encoder includes an adaptive codebook that generates an adaptive codevector representing a pitch component, a random codebook that generates a random codevector representing a random component, and a synthesis filter that generates a synthetic speech signal by being excited by the adaptive codevector and the random codevector. The random codebook includes an input vector provider configured to provide an input vector, and an excitation vector generator configured to generate an excitation vector as the random codevector by dispersing the input vector by using a fixed pattern. A length of the fixed pattern is shorter than a length of a sub-frame.