Musical noise is a typical problem with blind source separation using a time-frequency mask. Recently, the cepstral smoothing of spectral masks (CSM) was proposed. Based on the idea of smoothing in the cepstral domain, this paper proposes the cepstral smoothing of separated signals (CSS) on the assumption that a cepstral representation better reflects the characteristics of speech signals than those of masks (or filter gains). We also report a comparative evaluation study of CSM and CSS with other musical noise reduction methods. Our experimental results show that CSM is effective for musical noise reduction, but the target speech was relatively distorted. On the other hand, our proposed CSS produced less distorted target signals with the same musical noise reduction as CSM.