Such large disasters as earthquakes and hurricanes are very unpredictable. During a disaster, we must collect information to save lives. However, in time disaster, it is difficult to collect information which is useful for ourselves from such traditional mass media as TV and newspapers that contain information for the general public. Social media attract attention for sharing information, especially Twitter, which is a hugely popular social medium that is now being used during disasters. In this paper, we focus on the information sharing behaviors on Twitter during disasters. We collected data before and during the Great East Japan Earthquake and arrived at the following conclusions: - Many users with little experience with such specific functions as reply and retweet did not continuously use them after the disaster. - Retweets were well used to share information on Twitter. - Retweets were used not only for sharing the information provided by general users but used for relaying the information from the mass media. We conclude that social media users changed their behavior to widely diffuse important information and decreased nonemergency tweets to avoid interrupting critical information.