A rectangular substrate 12 composed of c-plane sapphire is prepared. Nickel serving as a catalytic metal is deposited on the entirety of an upper surface of the substrate 12 to form a catalytic metal film 14 (see (a)). The catalytic metal film 14 is patterned by a lithography method into a catalytic metal film 16 having a predetermined shape (see (b)). The temperature of the catalytic metal film 16 is raised to 1000° C. and maintained at 1000° C. for 20 minutes. The temperature of the catalytic metal film 16 is lowered from 1000° C. to 800° C. at a rate of 5° C./min. The temperature of the catalytic metal film 16 is maintained at 800° C. for 15 hours. Thereby, a catalytic metal layer 17 having large grains is provided (see (c)).