A life prediction method and device that can predict the life of a display device considering the differences between temperatures at measurements of the luminance of the display device. A monitor measures the luminance of the display screen using an optical sensor and measures the temperature around the display screen using a temperature sensor. A terminal device stores the measured luminances and temperatures in such a manner that the luminance and temperature are associated with each other. On the basis of the measured luminances and temperatures obtained by repeated measurements, the terminal device predicts the trend of changes in the luminance assuming that the temperatures at the measurements have been approximately constant, and predicts the life of the monitor on the basis of the predicted change trend.