It is used a substrate main body 1 having a side face 1b and a pair of main faces 1a and an underlying film 2 of a single crystal of a nitride of a metal belonging to the group III formed at least on one main face of the substrate main body 1. A single crystal 3 of a nitride of a metal belonging to the group III is grown on the main face 1a of the substrate main body 1 by a liquid phase process. The underlying film 2 has a shape of a convex figure in a plan view. A surface 4 without the underlying film thereon surrounds the entire circumference of the underlying film 2. The single crystal 3 of a nitride of a metal belonging to the group III grown on the underlying film 2 is not brought into contact with a single crystal of a nitride of a metal belonging to group III formed on another underlying film.