A far-infrared camera is thought to be well suited to detection of pedestrians at night. However, around sunset time during the summer, because the temperature difference between pedestrians and background objects becomes small, it is difficult to distinguish pedestrians with the far-infrared camera. To solve this problem, we propose a new contrast control method that is varying of the image sensor temperature. By means of increasing the image sensor temperature from 25 to 50°C the contrast between road and clothes, and sky and clothes increased about 1.54 and 1.64 times, respectively. The contrast between sky and skin increased about 1.55 times, but that between the road and skin did not increase. Because the emissivities of these objects were similar. The contrast control method is effective to detect objects with relatively large differences in emissivity at just about the same temperature.