We proposed a sensing system in order to watch for and to provide information about aged people in his room. Our system is based on measurement of indoor environmental changes (gas concentration, temperature, humidity, brightness, and so on) according to daily activities of residents. Measured data is used to recognize resident's activities, such as cooking, eating, sleeping, going out, bathing, and so on. In the aim of realizing those system, we developed a multi-sensor system with the following features. 1) Various types of sensor can be connected without any adjustment of interface circuit. 2) The exchange of sensors is easy. 3) The adjustment of the sensor sensitivity is easy. Our sensor system is effective to measure a object which is complex and which changes slowly, and to understand and evaluate the object like indoor environment in real world around us. We monitored indoor environmental changes using the system, and made evaluations on the ability of classification of daily activities.