Spreading rumors on the Internet has become increasingly pervasive due to the proliferation of online social media. This paper investigates how rumors are amplified by a group of users who share similar interests or views, dubbed as an echo chamber. To this end, we identify and analyze 'rumor' echo chambers, each of which is a group of users who have participated in propagating common rumors. By collecting and analyzing 125 recent rumors from six popular fact-checking sites, and their associated 289,202 tweets/retweets generated by 176,362 users, we find that the rumors that are spread by rumor echo chamber members tend to be more viral and quickly propagated than those that are not spread by echo chamber members. We propose the notion of an echo chamber network that represents relations among rumor echo chambers. By identifying the hub rumor echo chambers (in terms of connectivity to other rumor echo chambers) in the echo chamber network, we show that the top 10% of hub rumor echo chambers contribute to propagation of 24% rumors by eliciting more than 36% of retweets, implying that core rumor echo chambers significantly contribute to rumor spreads. Online media such as social networks, online communities, instant messages, and e-mails have become popular vectors in disseminating news, content, political campaigns, scientific findings, or product advertisements. Due to the nature of such online media that spreads information quickly and widely 1-3 , there have been attempts to disseminate misinformation, false news, or rumors, the last of which are circulating stories of uncertain or ungrounded gossips 4,5. Such rumors usually affect people, society, or economics. For example, the misinformation about Hurricane Sandy on the east coast of the USA brought the storm of rumors and false photos on the Internet 6. Another well-known widespread rumor is about the Boston Marathon Bombing event. FBI requested for materials related to the suspects (as bombers), but many people reported and propagated a deluge of misinformation or rumors to that event at the moment 7. A false news about Barack Obama's injury due to the White House explosion was reported through Twitter by the Associated Press (AP), which brought monetary chaos in stock markets-$130 billion in stock values were wiped out 8. The growing importance in understanding rumor spreads has spurred research into analyzing rumor propagation patterns in online media. By analyzing the factors that are related to rumor propagation, researchers have investigated how different types of content 9,10 or initial propagation patterns 9 are associated with the rumor propagation. From a user perspective, the relations between rumor propagation patterns and individuals' interest (or political standpoint) 11,12 , behavioral characteristics 7,13 , or social structures 14 have been investigated. While these studies have provided valuable insights into understanding rumor spreads and their associated factors, relatively little attention has been paid to how a group of users that share...