A thermal processing apparatus (1) comprises a chamber body (6), a holding part (7) for holding a substrate (9) inside the chamber body (6), a light emitting part (5) for heating the substrate (9) through light irradiation and a light measuring part (2) for measuring light energy. The light measuring part (2) comprises a calorimeter (24) disposed outside the chamber body (6), a light guide structure (20) for guiding the light inside the chamber body (6) to the calorimeter (24) and a calculation part (25) for performing computations on the basis of an output of the calorimeter (24). In the thermal processing apparatus (1), by measuring the light from the light emitting part (5) by the calorimeter (24), it is possible to measure the energy of light emitted from the light emitting part (5) during thermal processing inside chamber body (6) and obtain a surface temperature of the substrate (9) by the calculation part (25).