A support substrate is bonded to a GaN epitaxial substrate including at least an electron transport layer and an electron supply layer grown on a growth substrate in the Ga-polar direction such that the support substrate faces the Ga-plane of the GaN epitaxial substrate. Furthermore, at least the growth substrate is removed from the GaN epitaxial substrate so as to expose an N-plane of the GaN epitaxial substrate. Subsequently, a semiconductor element is formed on the N-plane side.