A method for forming an underlayer composed of a GaN-based compound semiconductor is provided. In this method, at the time of epitaxial growth of an underlayer on the surface of a sapphire substrate, no gap is generated between the underlayer and the surface of the sapphire substrate. The method for forming an underlayer composed of a GaN-based compound semiconductor includes the steps of forming strip seed layers composed of a GaN-based compound semiconductor on the surface of a sapphire substrate, forming a crystal growth promoting layer composed of a GaN-based compound semiconductor on the top surfaces and both the side surfaces of the seed layers, and on the exposed surfaces of the sapphire substrate, and epitaxially growing an underlayer composed of a GaN-based compound semiconductor from the parts of the crystal growth promoting layer.