Provided is a high quality III nitride semiconductor device in which, not only X-shaped cracks extending from the vicinity of the corners of semiconductor structures to the center portion thereof, but also crack spots at the center portion can be prevented from being formed and can provide a method of efficiently manufacturing the III nitride semiconductor device. The III nitride semiconductor device of the present invention includes a support and two semiconductor structures having a nearly quadrangular transverse cross-sectional shape that are provided on the support. The two semiconductor structures are situated such that one side surface of one of the two semiconductor structures is placed to face one side surface of the other of them. The support covers the other three side surfaces and of the four sides of the semiconductor structures.