We investigate the relation between the sentiment of a message on social media and its virality, defined as the volume and the speed of message diffusion. We analyze 4.1 million messages (tweets) obtained from Twitter. Although factors affecting message diffusion on social media have been studied previously, we focus on message sentiment, and reveal how the polarity of message sentiment affects its virality. The virality of a message is measured by the number of message repostings (retweets) and the time elapsed from the original posting of a message to its Nth reposting (N-retweet time). Through extensive analyses, we find that negative messages are likely to be reposted more rapidly and frequently than positive and neutral messages. Specifically, the reposting volume of negative messages is 1.2--1.6-fold that of positive and neutral messages, and negative messages spread at 1.25 times the speed of positive and neutral messages when the diffusion volume is large.