A speaker which is thin but which is capable of providing high-fidelity reproduction and ensuring a suitable revel of sound pressure even when it is brought close to a wall. A closed chamber is formed in the rear of a part of a diaphragm mounted in an enclosure, and an open chamber is formed in the rear of the other part of the diaphragm. The phase difference between sounds radiated from the front and rear surfaces of the diaphragm is increased as large as possible by using an acoustic duct for the sound radiated through the open cavity, thereby minimizing cancellation of sounds and improving the sound pressure level.