A variable geometry flame trap device for use in an after-burner device of a gas turbine, which is not exposed to deformation forces during its use in the after-burner mode. This is achieved with a plurality of flame trap plate pairs (21), positioned downstream of the turbine at the outlet of a primary air flow duct (9) and mounted so as to pivot on a plurality of diffuser arms (19) located at the outlet of a secondary air flow duct (7), characterized in that the plates (21) of each pair are made from a composite material and in that they have a cam projecting from their facing faces (42), said two cams (43) bearing on one another.