PURPOSE: To obtain an acoustic diaphragm having good acoustic characteristics by forming a thin film of a blank surface material on one surface of a base material, and then adhering the blank surface material to a core material and removing the base material.

CONSTITUTION: The thin film of the blank surface material 2 is formed on one surface of the base material 1. In this case, the base material 1 is easy to separate from the surface material 2. Then, hot-melt sheets 4 of heat fusing adhesives are disposed on both sides of the honeycomb core material 3 and further surface materials 1 are arranged on both sides of them. In this state, specific pressure is applied from both external sides of the base material 1 for a specific time and a heat treatment is carried out. Thus, the surface materials 2 are adhered to both surfaces of the core material 3. Then, only the base material 1 is peeled by a peeling method. Thus, the acoustic diaphragm of honeycomb sandwich structure having the surface materials 2 with large specific elasticity on both sides of the core material 3 with adhesive layers between is obtained.