An acoustic apparatus (1; 21) for resonating a stringed instrument (100; 600) having a sound box (103; 603) and a sound hole to emit a sound wave from the stringed instrument to a listener side, the acoustic apparatus comprising: a vibration generator (2; 22; 32); a support body (3; 23) for supporting the vibration generator; a locking portion (4; 24) which is configured to be locked directly to the stringed instrument or locked to a stand for holding the stringed instrument, and a spacer (6; 26), wherein the vibration generator has a diaphragm (201; 221; 231) for converting a sound driving signal into an acoustic vibration, and wherein, in a state in which the locking portion is locked directly to the stringed instrument or in a state in which the locking portion is locked to the stand and the stand is holding the stringed instrument: the vibration generator is arranged to face the sound box, the vibration generator is spaced apart from a front surface of the sound hole, the vibration generator is arranged to face the sound hole from an outside of the stringed instrument, and the spacer is arranged between the sound box and the support body so as to be in contact with a front plate of the stringed instrument for appropriately keeping a clearance between the vibration generator and the front plate, with the spacer extending between the support body and the front plate of the stringed instrument in which the sound hole is formed. a vibration generator (2; 22; 32); a support body (3; 23) for supporting the vibration generator; a locking portion (4; 24) which is configured to be locked directly to the stringed instrument or locked to a stand for holding the stringed instrument, and a spacer (6; 26), wherein the vibration generator has a diaphragm (201; 221; 231) for converting a sound driving signal into an acoustic vibration, and wherein, in a state in which the locking portion is locked directly to the stringed instrument or in a state in which the locking portion is locked to the stand and the stand is holding the stringed instrument: the vibration generator is arranged to face the sound box, the vibration generator is spaced apart from a front surface of the sound hole, the vibration generator is arranged to face the sound hole from an outside of the stringed instrument, and the spacer is arranged between the sound box and the support body so as to be in contact with a front plate of the stringed instrument for appropriately keeping a clearance between the vibration generator and the front plate, with the spacer extending between the support body and the front plate of the stringed instrument in which the sound hole is formed. the vibration generator is arranged to face the sound box, the vibration generator is spaced apart from a front surface of the sound hole, the vibration generator is arranged to face the sound hole from an outside of the stringed instrument, and the spacer is arranged between the sound box and the support body so as to be in contact with a front plate of the stringed instrument for appropriately keeping a clearance between the vibration generator and the front plate, with the spacer extending between the support body and the front plate of the stringed instrument in which the sound hole is formed.