A sound mixing apparatus includes mixing processing units 11 to 1k provided according to sampling frequencies and speaker detector 400 that selects at least one speaker sound signal out of plural sound signals inputted to mixing processing units 11 to 1k. Each of mixing processing units 11 to 1k adds up speaker sound signals having the same sampling frequency selected by speaker detector 400 to generate a first added-up sound signal, converts the sampling frequency of the first added-up sound signal into a sampling frequency processable by the other mixing processing units, and further adds up the first added-up sound signal and the first added-up sound signal converted into a sampling frequency processable by the processing unit and supplied from the other mixing processing units.