To provide a method to reduce generation of pyrolyzed products of a rock wool formed body that harmful pyrolyzed products are not produced even when a rock board is used at high temperature while keeping the low cost and good processing property of the board.

The method to reduce generation of pyrolyzed products of a rock wool formed body is aimed to reduce the pyrolyzed products produced when a rock wool formed body obtained by molding rock wool with a binder consisting of a phenol resin or its modified compound is used at high temperature. By the method, the rock wool formed body is heat treated under such heating condition that the binding force of the binder is not lost. The heating condition is preferably at 150 to 500°C heating temperature. The pyrolyzed products produced during the heat treatment are preferably detoxified by using an oxidation catalyst disposed in the ventilation passage of the furnace.