Since the traffic of NetNews is increasing, storage of all articles becomes a serious problem from the viewpoint of wasting network bandwidth and the amount of disk usage. In addition, not all incoming articles are read by users. We have proposed several caching algorithms to overcome this problem and shown that a selective prefetch scheme gives the best system performance among the proposed ones. However, since the selective prefetch scheme employed a simple prefetching policy, the scheme gave a low hit ratio in some cases. Therefore, the paper improves the selective prefetch scheme from the viewpoint of the amount of disk usage as well as hit ratio. We divide the scheme into three factors: reference span, criterion, and threshold in criterion. Through simulation experiments using actual NetNews logs, we investigate the influence of the factors of the reference span and the threshold to system performance. As a result, it is shown that the reference span is a more significant factor than the threshold and when the selective prefetch scheme has a value of around seven days, reference span keeps a high hit ratio and reduces the amount of disk usage.