Abstract: A filter medium forming system for forming a filter medium of the type wherein pleats radially outwardly extends in which sides of the pleats are alternately bonded at the inner and outer peripheral sides to allow fluid to be filtered flows generally along the length of the pleats. The filter medium forming system is comprised of a scoring device for forming linear depressions (score lines) on the opposite side surfaces of an elongate sheet-type filter medium. The scoring device is followed by an adhesive applying device for applying adhesive onto the opposite side surfaces of the filter medium at the edge sections. The adhesive applying device is followed by a pleating device for pleating the filter medium along the linear depressions to form a pleated filter medium having a plurality of pleats each having two sides contiguous through each linear depression.