To provide an under-floor thermally insulating plate for a vehicle reducing a thermal influence to a part of center bearing insulator, rubber coupling, etc., arranged in a space in a floor tunnel, by heat convection of air heated by an exhaust pipe in the floor tunnel and an exhaust pipe system of catalytic converter or the like.

In a catalytic converter thermally insulating plate 7 arranged in the vicinity of a catalytic converter 8 in an under-floor lower surface of a vehicle to be provided with a surface 7a covering an upper surface of the catalytic converter and surfaces 7b, 7c covering both side surfaces thereof, this constitution comprises a surface 7d closing partly a rear end part of an opening formed by the surface 7a covering the upper surface and the surfaces 7b, 7c covering both the side surfaces and a plurality of ribs 7e in the vicinity of a front end part of a surface in the ground surface side of the surface 7a covering the upper surface.