We present a simple interferometric thermometry for non-contact temperature measurement of silicon substrates, When pulse-modulated light from an InGaAsP infrared laser diode was directed at a silicon wafer polished on both sides, overshoot and undershoot behaviors in the transmitted light intensity were observed. These phenomena were utilized to determine the direction of temperature change. We measured the temperature of a silicon wafer under successive heating and cooling. This temperature monitor was able to measure up to 600 degrees C with 1.3 mu m wavelength laser and 700 degrees C with a 1.55 mu m one. By shortening the cycle time of the pulse modulation, this method can follow temperature changes as fast as 450 degrees C/s for 0.5-mm-thick silicon wafers.