A mixing apparatus having a stereo output includes: a first signal processor that mixes a first signal and a second signal in a first channel; a second signal processor that mixes a third signal and a fourth signal in a second channel; a third channel that processes a weighted sum of a signal of the first channel and a signal of the second channel; and a gain deriving part that generates a gain mask commonly used in the first channel and the second channel, wherein the gain deriving part determines a first gain commonly applied to the first signal and the third signal, and a second gain commonly applied to the second signal and the fourth signal, so that predetermined conditions for simultaneous gain generation are satisfied at least at the first channel and the second channel among the first channel, the second channel, and the third channel.