A sound masking device includes: a sound pick-up unit which picks up a sound, and which produces a picked-up sound signal based on the sound, a masking sound signal producing unit which produces a masking sound signal from the picked-up sound signal a sound emitting unit which emits a masking sound based on the masking sound signal, and an echo cancelling unit which performs an echo cancelling process on the picked-up sound signal by performing an adaptive filtering process on the masking sound signal, and by subtracting the filtered masking sound signal from the picked-up sound signal. The masking sound signal producing unit produces a masking sound signal having a level corresponding to an amplitude level of the picked-up sound signal which has subjected to the echo cancelling process.