PURPOSE: A method of manufacturing a composite substrate for vehicles using extruded sheets is provided to reduce the weights of vehicles and to improve the sound absorbing and heat insulating property of a composite substrate by including a light foamed sheet in a core unit. CONSTITUTION: A composite substrate (10) for vehicles comprises a foamed sheet (5) based on a resin selected among a polypropylene-based resin, a polyethylene-based resin, and a mixture thereof. Extruded sheets (3a,3b), which are prepared by dispersing glass fibers (7) in a polypropylene-based resin, are thermally bonded on both sides of the foamed sheets. A method of manufacturing the composite substrate comprises the steps of: injecting a foamed substance material composition containing a foaming agent and a resin selected among a polypropylene-based resin (9), a polyethylene-based resin, and a mixture thereof into an extruder; melt-extruding a first extruded sheet; injecting a raw material composition, in which glass fibers are dispersed in a polypropylene-based resin; melt-extruding second extruded sheets; laminating and thermally bonding the second extruded sheets on both sides of the first extruded sheet; and foaming the first extruded sheet by foaming the foaming agent.