PURPOSE: To obtain the title filter medium which has high filter precision and large filtration flow rate by manufacturing fiber made of organic material into sheets to obtain bandlike filter paper, and overlapping this bandlike filter paper and a bandlike reinforcing sheet having rigidity of a degree capable of fluting in both end parts in the width direction thereof, melt-sticking and joining both and molding them into flutes.

CONSTITUTION: Fiber made of organic material is manufactured to sheets to obtain filter paper 1. This filter paper 1 and a reinforcing sheet 5 described hereunder are overlapped. Both end parts of filter paper 1 are thermally melt-stuck and joined to the reinforcing sheet 15. This reinforcing sheet 5 equips fluid-permeable performance and has rigidity of a degree capable of fluting. The joined and integrated filter medium sheet (filter paper 11 and reinforcing sheet 15) is sent between the bent curve forming rollers 25, 25 mutually oppositely arranged. The bent curves 10a are formed at the prescribed intervals by a plurality of riblike teeth 25a which are lengthened to the axial direction in the equal intervals on the outer peripheral surface of the upside roller 25. In such a way, filtration performance of filter paper can intactly be maintained and moreover handling of filter medium 10 is made easy by producing the filter medium 10 which is formed of filter paper 11 and the reinforcing sheet 15 and molded into flutes.