A right expected value generator generates an expected value of a right channel spectrum from the right channel spectrum. Further, a left expected value generator generates an expected value of a left channel spectrum from the left channel spectrum. Further, a right channel spectrum corrector so corrects a right channel spectrum outputted from a second synthesizer that the right channel spectrum does not exceed the expected value of the right channel spectrum. Moreover, a left channel spectrum corrector so corrects a left channel spectrum outputted from the second synthesizer that the left channel spectrum does not exceed the expected value of the left channel spectrum.