A graphite substrate is accommodated into a chamber where vacuum drawing is performed via a pump. Thereafter, carbon is heated under vacuum, whereby impurities in the carbon are evaporated causing the carbon to be purified. The carbon in the graphite substrate is purified, achieving suppression of the impurities as donor/acceptor elements and also metallic elements in the semiconductor layer of 0.1 ppm or less, the impurities being contained in the carbon in the graphite substrate. As a result, occurrence of leak current or an abnormal leak point enables to be suppressed, and thus abnormal crystal growth in the semiconductor layer enables to be suppressed.