近年,Twitter をはじめとするマイクロブログを利用した商品やイベントの告知に対し,多くのユーザが感想など告知の投稿者やその告知への感想に関心があるユーザにとって有益な関連投稿を行うようになってきている.しかし,関連投稿の多くは告知投稿とは明示的に関連付けられてはいないため,告知の投稿者がこれらの関連投稿を見つけるのは容易ではない.そこで本研究では,特に Twitter の機能であるリツイートに注目し,告知に対する非明示的な関連投稿を効率的に収集する手法を提案する.Events, festivals or product releases are often announced via microblogs such as Twitter. Many users post messages that are relevant to the announcements. Such relevant posts are useful for both the authors of the announcements and the users who are interested in public opinions to the announcements. However, it is not easy to collect such relevant posts because many of the relevant posts are not explicitly associated with the announcement post. In this paper, we propose a method for efficiently collecting such posts that are only implicitly relevant posts to announcements, focusing on retweets of Twitter in particular.