This paper describes enhancement of the spatial memory system in order to improve human activities observation. In previous approach, human activities using the spatial memory in the real world can be observed, but activities using physical objects can not be observed. Therefore, we will focus on interaction among humans and objects. By attaching small sensor nodes to humans' hand and physical objects, the observation will be achieved. In order to realize uniformed methodology for the observation of human activities based on the spatial memory, we introduce Object-based Spatial- Knowledge-Tags (O-SKTs). History of human-object interaction is described by using an O-SKT including 4W1H (who, what, when, where and how) information. This paper presents a system to observe human-object interaction and experimental results of the observation.