PURPOSE: To eliminate the need for a bonding process by overlapping a thermoplastic reinforcing net whose single face is coated with a thermoplastic adhesive on a filter stock and folding both and thereafter thermally bonding both to produce a filter material.

CONSTITUTION: As a thermoplastic reinforcing net 2 which is overlapped on a filter stock 1 such as nonwoven fabric and integrated into one body, the material whose single face is coated with a thermoplastic adhesive is sued. The filter stock 1 and the reinforcing net 2 are overlapped and folded by a folding machine 5 and thereafter respective top parts 13 are pressed on the hot plates 7 while passing through the hot plates 7 and folding is set and simultaneously both are thermally bonded and thereby the filter stock 1 and the reinforcing net 2 are partially into one body. As a result, a process for bonding and integrating the filter stock 1 and the reinforcing net 2 before the folding process can be omitted.