A copying machine of the diazo type comprises a light exposure zone for exposing to light a diazo type photosensitive sheet superposed with an original, a development zone for developing the light exposed photosensitive sheet and a delivery mechanism for feeding the original and diazo type photosensitive sheet to the light exposure zone and forwarding the light exposed photosensitive sheet to the development zone. The light exposure zone includes a transparent cylinder capable of rotating while having a superposed assembly of the original and photosensitive sheet closely contacting the outer surface thereof and a heat-generating light source mounted in the interior of the transparent cylinder. The transparent cylinder has a fan for feeding cold air into the transparent cylinder from one end thereof and cooling the outer surface of the transparent cylinder substantially uniformly along the entire length thereof and a temperature-detecting mechanism thermoconductively contacting the outer surface of the transparent cylinder to detect the temperature of the outer surface. A temperature-controlling mechanism is provided to control the air feed rate of the fan in response to signals from the temperature detecting mechanism, to control the temperature of the outer surface of the transparent cylinder within a predetermined temperature range.