To build up spontaneous conversation, it is important to select topics without a feeling of strangeness. When someone notices others are not interested in a topic, he/she tries to find a new topic. Then, he/she thinks of viewpoints of the conversation and selects a topic associated with the current topic from the viewpoints. To automate viewpoint-based topic selection, we present 5W viewpoint associative topic search. The method estimates the weights of 5W viewpoints (who, what, where, when and why) from conversation, to use an appropriate similarity to search for the next topic.