1. A digital watermark embedding device comprising: a synthesized voice generating unit that outputs a synthesized voice according to an input text and outputs phoneme-based alignment regarding phonemes included in the synthesized voice; an estimating unit that estimates whether or not a potentially risky expression is included in the input text, and outputs a potentially risky segment in which the potentially risky expression is estimated to be included; an embedding control unit that associates the potentially risky segment with the phoneme-based alignment, and decides and outputs an embedding time for embedding a watermark in the synthesized voice; and an embedding unit that embeds a digital watermark in the synthesized voice at a time specified as the embedding time for the synthesized voice.