PURPOSE: To improve the productivity, by bonding a surface member on both sides of a flat core made of a thermoplastic resin, having radial rugged rib and a flange at the outer circumference and providing a drying small hole for drying a bonding paste.

CONSTITUTION: The flat core member 4 is formed with the thermoplastic resin film by using a means such as press forming, vacuum forming or compression forming, and has radial rugged ribs 4', 4" and is provided with a flange 4''' at the outer circumference. After the core member 4 is formed with the forming, a rectangular flat speaker diaphragm 6 is formed by sticking surface members 5, 5' made of a light metal foil having comparatively some elasticity on both the sides of the core member 4 having the radial rugged ribs 4', 4". The diaphragm manufactured in this way has a good productivity and is low in the cost. Further, the bonding paste of the surface member 5' is dried by providing an air vent 7 to one side (projected side) 4" of the flat core member 4, allowing to improve much more the mass-productivity.