Abstract: Provided are a process for producing a compatibilized resin including reacting a cyanate compound, a siloxane resin having a hydroxyl group at an end thereof, and an epoxy resin at a specified rate of reaction; a thermosetting resin composition containing (A1) a compatibilized resin produced by the foregoing process or (A2) a thermosetting resin obtained by reacting a cyanate compound and a siloxane resin having a hydroxyl group at an end thereof at a specified rate of reaction, and (B) fused silica having been subjected to a surface treatment with a trimethoxysilane compound; and a prepreg, a laminate, and a wiring board each using the same.