A speech signal decoding device includes an adaptive code-book decoder, a fixed code-book decoder, a ratio calculator, a determination unit, and an attenuator. The adaptive code-book decoder generates an adaptive code-book vector using an adaptive code-book code of an encoded signal encoded by a CELP method. The fixed code-book decoder generates a fixed code-book vector using a fixed code-book code of the encoded signal. The ratio calculator calculates an amplitude ratio or an energy ratio between the adaptive code-book vector and the fixed code-book vector. The determination unit determines whether the amplitude ratio or the energy ratio exceeds a predetermined threshold. When the amplitude ratio or the energy ratio is determined to exceed the predetermined threshold, the attenuator attenuates an excitation signal that has been defined by adding the adaptive code-book vector to the fixed code-book vector.