To produce a polydiorganosiloxane/silica mixture having excellent adhesiveness by mixing a modified silica filler obtained by heating a mixture comprising reinforcing silica and a surface modifier represented by a specified formula under agitation with a polydiorganosiloxane.

A reinforcing silica filler having a surface area of 90-500m2/g is mixed with 0.1-0.5 pts.wt., per pts.wt. silica filler, surface modifier represented by the formula: R1O(R2SiO)xR1 [ R is methyl, ethyl, vinyl or the like; R1 is methyl, ethyl or acetoxy; and x is 2-12] at 90-180°C, and the mixture is devolatilized to form a modified silica filler. Next, a polydiorganosiloxane having a viscosity of 0.1-400 Pa.s is added to the modified silica filler, and the resulting mixture is agitated for shorter than 10min to obtain a modified silica-polydiorganosiloxane mixture having a silica filler content of 25-40wt.%. This mixture is mixed with a cross-linking agent (e.g. methyltriacetoxysilane) and a curing catalyst (e.g. dibutyltin diacetate) to obtain a room-temperature- curing silicone sealant.