This paper describes the cyclic temperature variation beneath the rake face of a cutting tool in end milling. A newly developed infrared radiation pyrometer equipped with two optical fibers is used to measure the temperature. A small hole is drilled in the tool insert from the underside to near the rake face, and an optical fiber is inserted in the hole. One of the optical fibers runs through the inside of the machine tool spindle and connects to the other optical fiber at the end of the spindle. Infrared rays radiating from the bottom of the hole in the tool insert during machining are accepted and transmitted to the pyrometer by the two optical fibers. For a theoretical analysis of the temperature in end milling, a cutting tool is modeled as a semi-infinite rectangular corner, and a Green’s function approach is used. Variation in tool-chip contact length in end milling is considered in the analysis. Experimentally, titanium alloy Ti–6Al–4V is machined in up and down milling with a tungsten carbide tool insert at a cutting speed of 214 m/min. In up milling, the temperature beneath the rake face increases gradually during the cutting period and reaches a maximum just after the cutting. In contrast, in down milling, the temperature increases immediately after cutting starts; it reaches a maximum and then begins to decrease during cutting. This suggests that the thermal impact to the cutting tool during heating is larger in down milling than in up milling, whereas that during cooling is larger in up milling than in down milling. Temperature variation is measured at different depths from the rake face. With increasing depth from the rake face, the temperature decreases and a time lag occurs in the temperature history. At 0.6 mm from the major cutting edge, the temperature gradient toward the inner direction of the tool insert is about . The calculated and experimental results agree well.