In context-aware computing, the context aggregation is an important function of the context management. In an infrastructure-based smart space, a centralized context management system need not concern about its resource consumption for context aggregation. However, in a personal smart space which consists of only resource-constrained mobile devices, not only global resource consumption of the personal smart space but also that of the device which plays a role of a context manager (coordinator) must be minimized. In this paper, we propose a task decomposition scheme in which heavy context aggregation tasks to be imposed on a centralized coordinating device are decomposed and distributed to all the participating mobile devices (clients) in a mobile smart space. By decomposing and distributing the heavy aggregation operations the processing overhead upon the coordinating device can be minimized while providing equivalent context aggregation capability for applications, but maintaining the total amount of processing of all devices not to be significantly increased.