To stick a soundproof sheet for soundproofing in contact with a vehicle body on a synthetic resin foam without using adhesive.

A soundproof sheet 10 having a large METSUKE amount is set to a predetermined position of a mold 9. Heated and foamed synthetic resin foam 5 is introduced between molds 8 and 9 in the state that the foam 5 is maintained at a heating temperature. The molds 8, 9 are temperature-regulated at 20 to 50°C, and pressed. The resin of the foam 5 at high temperature is impregnated with backing material 6 having small METSUKE amount to reach the sheet 10, and hence the sheet 10 is integrated with the foam 5.