Pressure in a chamber receiving a semiconductor wafer is reduced to a level less than atmospheric pressure. The semiconductor wafer is subjected to heat treatment in a reduced-pressure atmosphere by being irradiated with a flash of light. A leak determination part determines that a leak occurs at the chamber if pressure in the chamber does not reach target pressure while a time period passed since start of reduction of pressure in the chamber exceeds a threshold set in advance. A leak at the chamber is detected by monitoring a time period passed since start of reduction of pressure in the chamber. This makes it possible to determine the presence or absence of a leak at the chamber with a simple structure without requiring installation of a new hardware structure.