For forming a sound absorption/insulation honeycomb panel by stacking an air-permeable material, a honeycomb material filled with a sound absorption material and a reflector, and adhesively joining these materials, it is hard to join the honeycomb material and the air-permeable material adhesively due to a thin wall surface of the honeycomb material and a resultant line to surface adhesive joint therebetween, causing a problem of low adhesive strength. By using a water absorption honeycomb material, an adhesive joint is formed with an adhesive joint area increased by dipping an end of a wall surface of a cell forming the water absorption honeycomb material into a water-soluble adhesive, making the end flexible over a fixed period of time, and then pressing the end strongly against an air-permeable material as a counterpart of the adhesive joint to deform a tip into an inverted T-shape.