Composite substrate manufacturing method, and composite substrate

Disclosed is a composite substrate manufacturing method whereby, after bonding a semiconductor substrate (1) and a supporting substrate (3) to each other, the semiconductor substrate (1) is thinned, and a composite substrate (8) having a semiconductor layer (6) on the supporting substrate (3) is obtained. On the supporting substrate (3) surface to be bonded, a coating film (4a) containing polysilazane is formed, a silicon-containing insulating film (4) is formed by performing firing by heating the coating film (4a) to 600-1,200° C., then, the semiconductor substrate (1) and the supporting substrate (3) are bonded to each other with the insulating film (4) therebetween, thereby suppressing bonding failures due to surface roughness and defects of the supporting substrate, and easily obtaining the composite substrate.