PURPOSE: To provide a quick thermal oxidizing apparatus for a semiconductor substrate having a high temperature measuring accuracy by using a radiant pyrometer.

CONSTITUTION: When a semiconductor substrate 1 is quickly heated by using a plurality of heating lamp groups 3, 4 so arranged as to perpendicularly cross to each other oppositely to upper and lower surfaces through the substrate 1 and a temperature of the upper surface of the substrate 1 is measured by a radiant pyrometer 7 through an opening 11 opened at a reflecting plate 5 of the upper surface side, a radiant light from the group 4 of the lower surface is shielded by a shielding shutter 8 openably disposed between an opposite measuring surface of the substrate and the group 4 of the lower surface, thereby measuring the surface temperature of the substrate 1 in a high accuracy.