The widespread of word of mouth using retweets on Twitter has enabled us to estimate trends in the real world. Previous research methods estimate the value of a tweeted content by calculating the number of subscribers who receive the tweet. However, we should consider the numbers of followers for both the tweeter and retweeter(s) as a greater number of followers may result in more retweets, which we call "bias." In this paper, we propose a bias-free evaluation method for tweeted contents. Experiments show that our method is successful at evaluating tweets without biases.