A large number of semi-structured documents (HTML documents) exist on the Web. To improve the accessibility of information related to an object, such as a product, human, or place, its attribute names and attribute values need to be extracted. Recently, many studies have aimed to extract attribute values automatically, whereas only a small number of studies have attempted to extract attribute names. Thus, we propose a method for extracting attribute names automatically. For the first time, we apply a bootstrapping algorithm to attribute name extraction in the area of information extraction. To solve a problem that is caused by applying a pure bootstrapping algorithm to attribute name extraction, we use keys, which are also extracted by a bootstrapping algorithm. We found that using extracted keys improve the accuracy of attribute name extraction.