PURPOSE: To simplify the coupling type of a filter diaphragm and a casing and to prevent the leakage between both by integrally constituting the casing and covering this casing with plastic to be cured after injection molding of the filter diaphragm, then fixing the filter diaphragm to this plastic.

CONSTITUTION: A molding tool consisting of a stationary tool half part and a movable tool half part is used and the filter diaphragm 6 is directly blanked by the mold member of the molding tool constituted as a blanking punch. The blanked component is positioned within the mold hollow chamber in the direct molding tool. The flowable plastic material is then injected into the mold hollow chamber. The molding is released from the mold after curing of the plastic material. The small filter which consists of the integral casing 1 having an inflow part 2 and an outflow part 3 and in which the filter diaphragm 6 is fixed to the plastic constituting the casing 1 at the shoulder 4 on the inner side of the casing 1 in its marginal part range 6II is obtd.