A diaphragm (1) of a light sound converter which has a high performance, is suitable for mass-production and has a slit (1s) formed therein. A light emitting element and a light receiving element are disposed at positions opposed to a reflective part (1a) of the diaphragm (1) having a pressure receiving surface part (1d) and the reflective part (1a), and light is radiated from the light emitting element to the reflective part (1a) of the diaphragm (1) and the reflected light from the reflective part (1a) is received by the light receiving element, whereby the position of the diaphragm (1) can be detected.