Although there has been a great deal of research on automatic summarization, most methods are based on statistical approach, disregarding relationships between extracted textual segments. To ensure sentence connectivity, we propose a novel method to extract a set of comprehensible sentences that centers on several key points. It generates a similarity network from documents with a lexical dictionary, and applies spreading activation to rank sentences. We show evaluation results of a multi-document summarization system based on the method, participating in a competition of summarization, TSC task organized by the third NTCIR project.