Recessed section is formed in the rear surface of a speaker box body with a speaker mounted to the inner front surface of the body box. Circuit unit, such as a mixer, including operators is removably accommodated in the recessed section. There is also provided a retention structure for retaining the accommodated circuit unit in the recessed section. When the speaker is to be used, for example, the circuit unit can be removed from the speaker box body and placed near a human player, so that the human player is allowed to, for example, readily operate the operators of the circuit unit. The recessed section may be covered with a lid, in which case speaker-related accessories can be stored in the recessed section.