PURPOSE: To provide a rigid filter medium sheet not causing the peeling of a knitted net heat-stuck to one side of the membranous body from the body even under severe service conditions, ensuring high work efficiency at the time of pleating and not reducing the effective filtration area.

CONSTITUTION: A knitted net 3 made of fluororesin and having valleys 5 every two or more crossing parts 4 in the longitudinal direction and/or the transverse direction is laminated on one side of a porous filter membrane 2 made of fluororesin and the net 3 is heat-stuck to the membrane 2 at the crossing parts 4 in a linear contact state to obtain the objective filter medium sheet.