Because of the increasing number of high-rise buildings and large glass facades and the resulting problem of safe and effective maintenance, many studies have been going on progress to develop automated window cleaning systems in the last few years. But the research has not been going on progress to make decision the additional cleaning through the diagnosis of contaminant degree on outer window before or after cleaning. So, the sensor system is proposed to measure the contamination of building's outer-window in this paper. To check the cleaning performance of the window, it is mounted on a Building Maintenance Robot (BMR) which is automated window cleaning system. For detecting the contaminant level of outer window, the sensor module using the IR sensor is proposed. In order to apply the proposed sensor module to BMR system, the mechanism for the horizontal or vertical movement of the sensor module is designed. To improve the diagnostic accuracy of contaminant level, The Algorithm for position control using the proximity sensor is developed to keep a certain distance between sensor and window at the horizontal mechanism. A method detecting the contaminant level is verified by lab test. To validate the mechanism of the proposed sensor module, the field test is executed with the BMR system equipped with the sensor module.