In order to control the source temperature during vacuum evaporation, a device for measuring the source temperature with a photo-transistor (Ph. Tr.) has been developed. The source temperature ranging from 600°C to 1100°C could be measured with error smaller than 5°C. The source temperature during evaporation of Al was measured continuously and moreover the instantaneous change of the source temperature accompanied by melting Al could be observed. It is considered to be possible to control the source temperature in constant with this device.