In the blogosphere, there exist posts relevant to a particular subject and blogs that show interests in the subject. In this paper, we define a set of such posts and blogs as "blog community" and propose a method for extracting the blog community associated with a particular subject. The proposed method is based on the idea that the blogs who have performed actions to the posts of a particular subject are the ones that have interests in the subject, and that the posts which have received actions from such blogs are the ones that contain the subject. The proposed method selects a small number of seed posts that contain the subject. Then, it selects the blogs that perform actions to the seed posts over some threshold and the posts that have received actions over some threshold. By repeating these two steps, it gradually expands the blog community. The experimental results show that the proposed method exhibits a higher level of accuracy than the methods proposed in prior research.