A temperature measurement method comprises a step (A) of lighting a light source part to irradiate a substrate to be treated that is an object to be heated with light for heating, the light source part including a plurality of semiconductor light-emitting elements that emits light having a main emission wavelength range of 0.3 μm or more and less than 0.5 μm; a step (B) of turning off the light source part after the step (A); a step (C) of maintaining an unlit state of the light source part after the step (B); and a step (D) of measuring, during the step (C), a temperature of the substrate to be treated through observation of light emitted from the substrate to be treated using a thermometer having a sensitivity wavelength range different from the main emission wavelength range of light emitted from the light source part.