A speaker includes a diaphragm formed as an elongated box-shaped five face body having one open face; an edge for supporting the diaphragm so as to enable vibration of the diaphragm; a voice coil wound around and fixed to four side faces which are among the five faces of the diaphragm and which are adjacent to the open face; and a magnetic circuit for supplying a drive force to the voice coil. The diaphragm is configured such that a height from the open face to an upper face opposed to the open face is greater than or equal to twice a thickness of the voice coil, and a length of a long side of the upper face is greater than or equal to twice a length of a short side of the upper face, and the upper face and two side faces of the diaphragm define a long side direction of the diaphragm. Additionally, reinforcing ribs are formed as recessed and projecting shapes on the upper face and the two side faces of the diaphragm.