An X-ray CT apparatus includes an X-ray source configured to generate an X-ray a scintillator configured to convert the X-ray into a fluorescent a substrate including a plurality of photosensitive elements configured to convert the fluorescent into an electric charge a temperature sensor formed on the surface of the substrate a heat element formed on the surface of the substrate and a controller configured to control a temperature of the photodiode by adjusting an electric current of the heat element.