This paper describes the temperature variations observed in the drilling of carbon fiber-reinforced plastic (CFRP)/Al, CFRP/Ti, Al/CFRP, and Ti/CFRP stacks. An infrared radiation pyrometer equipped with an optical fiber was used to measure the temperature. The optical fiber, inserted into the oil hole of an internal-coolant carbide drill, registered the infrared rays radiating from the bottom surfaces of the drilled holes. In drilling the CFRP/Ti stack, the temperature was ∼95°C in the CFRP layer. As the drill progressed into the Ti layer, the temperature increased to a very high value of 745°C at the bore exit. In the Ti/CFRP stack, meanwhile, the initial temperature was ∼170°C and increased to 695°C at the Ti/CFRP interface. Severe thermal damage, including fiber/resin pullout and matrix degradation, was caused by the heat at the bottom surface of the drilled hole in the Ti/CFRP stack. Along the edge of the entry point in the CFRP, the CFRP matrix was degraded by the heat. In drilling the CFRP/Al stack, the temperature in the Al layer was 200°C; little thermal damage was observed.