Whereas a high masking effect can be secured in a space to which a masking sound is emitted, the degree of a discomfort a person existing in the space suffers can be reduced. In superimposition processing, a CPU 21 extracts sound signals in different intervals of a sound signal X12-n of a human voice, superimposes the extracted sound signals on each other on the time axis, and outputs a resulting superimposed sound signal X13-n. In shift and addition processing, the CPU 12 interchanges a sound signal, before a reference position, of a sound signal X16-n and a sound signal, after the reference position, of the sound signal X16-n (shift processing) and outputs a sound signal X17-n obtained by adding together a shift-processed sound signal X16'-n and the original, non-shift-processed sound signal X16-n.
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