A slim speaker includes a casing having an enlarged front opening and a rear end with a magnetic circuit mounted thereto; a voice coil located over a central area of the magnetic circuit; a diaphragm located between the enlarged front opening of the casing and the voice coil; and a damper located behind the diaphragm to space from the latter. The diaphragm has at least one V-shaped bent section provided at a middle portion for bearing on a corresponding position on the damper. With these arrangements, the diaphragm not only has an effectively reduced overall height to largely reduce the assembled volume of the slim speaker and allow the slim speaker to be easily mounted in the limited inner space of an electric product, but also stabilizes the vibration of the voice coil to enable good resonance of sound.