PURPOSE: To provide a filter medium for high precision filtration undergoing no change in the max. pore diameter even when sterilization by heating or filtration at a high temp. is carried out, having high tensile strength and capable of working such as folding.

CONSTITUTION: Low m.p. superfine fibers and high m.p. superfine fibers whose m.p. is higher than that of the low m.p. superfine fibers by ≥10°C are subjected o hot melt bonding to each other through the low m.p. superfine fibers and the resulting nonwoven fabric consisting of 20-80wt.% of the low m.p. superfine fibers and 80-20wt.% of the high m.p. superfine fibers is subjected to hot melt bonding to a hot melt bondable net to produce the objective filter medium having ≤120μm max. pore diameter.