A nitride semiconductor element capable of accommodating GaN electron transfer layers of a wide range of thickness, so as to allow greater freedom of device design, and a nitride semiconductor element package with excellent voltage tolerance performance and reliability. On a substrate, a buffer layer including an AlN layer, a first AlGaN layer (with an average Al component of 50%) and a second AlGaN layer (with an average Al component of 20%) is formed. On the buffer layer, an element action layer including a GaN electron transfer layer and an AlGaN electron supply layer is formed. Thus, an HEMT element is constituted.