A diaphragm in one mode of the present invention comprises an outer shape section having a circle with one diameter, or a shape of a polygon to be inscribed in a circle with the one diameter; an inner shape section protruding in one direction with regard to a plane including the outer shape section, wherein a shape formed by projecting the inner shape section on the plane is rotationally symmetric about the central axis of the circle; and a vibrating face section linking the outer shape section and the inner shape section and having an inclined face inclined to the central axis, wherein any one of cross-sectional shapes of the vibrating face section is rotationally symmetric about an eccentric axis which is eccentric with regard to the central axis, the cross-sectional shapes being defined by a plane intersecting perpendicularly to the central axis. An electroacoustic transducer uses the above diaphragm.