To provide method capable of grasping, rapidly and correctly, the state of a setting reaction, and capable of providing friction material having excellent quality, by measuring, stably on a real time, a reaction gas quantity generated during a thermoforming process of the friction material containing a thermosetting resin.

A fence 7 for preventing sudden diffusion of gas 3 generated by a reaction between a thermosetting resin and a setting agent, to the outside of a die assembly 1 at the time of degassing, is installed on the die assembly 1 for thermoforming of friction material (preliminary formed body 2) containing the thermosetting resin, and the generated reaction gas 3 in the fence 7 is measured by an optical measuring means (FT-IR4) to measure a change of an ammonia gas 4 quantity as an instant response. A beam of an optical measuring means is transmitted into the fence 7 of the die assembly 1 to measure directly the gaseous ammonia 4 quantity in the fence 7.