A process for forming a microstructure of a nitride semiconductor including (1) preparing a semiconductor structure which has a second semiconductor layer formed of a group III nitride semiconductor containing at least Al formed on a principal plane of a first semiconductor layer formed of a group III nitride semiconductor containing no Al, and which has a hole that penetrates through the second semiconductor layer and is formed in the first semiconductor layer; (2) subjecting the semiconductor structure to heat treatment under a gas atmosphere including a nitrogen element after step (1) to form a crystal plane of the group III nitride semiconductor containing no Al, on at least a part of a side wall of the hole; and (3) forming a third semiconductor layer formed of a group III nitride semiconductor on the second semiconductor layer after step (2) to cover the upper part of the hole.