Characteristics of a semiconductor device are improved. A semiconductor device includes a sequential stack of a buffer layer, a channel layer, and a barrier layer, and includes a mesa part including a fourth nitride semiconductor layer formed over the stack, and a side part formed on both sides of the mesa part and including a thin film part of the fourth nitride semiconductor layer. Generation of 2DEG is suppressed below the mesa part while being unsuppressed below the side part. In this way, the side part that disables the 2DEG suppression effect is provided on an end portion of the mesa part, thereby a distance from an end portion of the side part to the gate electrode is increased, making it possible to suppress leakage caused by a current path passing through an undesired channel formed between a gate insulating film and the mesa part.