A temperature measurement system with a reflected laser beam that does not require temperature modulation was developed for the thermal design of electronic devices. The system combines a balanced photodetector and lock-in amplifier to make reflectance change measurements possible with an extremely high signal-to-noise ratio. First, to evaluate the accuracy of the reflectance change measurement, the reflectance change of a Ni thin film temperature-controlled by a heater and thermocouples was measured as a function of temperature change. To evaluate the accuracy of the temperature change measurement, the reflectance change was measured by heating a Ni thin film that imitated the wiring pattern of an electronic circuit. The experimental results showed that the standard deviation of the ratio of reflectance change from the reference reflectance was <1.00 × 10−5 at a constant temperature. Moreover, the error margin of the calculated temperature change was ±1.0% for a change of 22 °C or more from the reference temperature, and the performance was sufficient for the estimation of local temperature.