The parallel downloading is one of the technologies which fetch a file at high speed. This technology retrieves each divided piece from a different server in parallel. On the other hand, we have proposed a proxy server which distributes the most recent files. As the technology that combines those, we have proposed a proxy server which gets the most recent files at high speed by using the parallel downloading technology. When this proxy server retrieves pieces, it must buffer them because of unordered arrivals so that it can provide a client with them in order. After that, the pieces are reordered and sent to a client. However, this proxy server has the problem that the buffer might become too big. We proposed in our previous study a substitution download method which alleviate the buffer growth, while it generates redundant traffic in some cases because of asynchronous retrieval of fixed sized blocks. In order to solve the problem, we propose a parallel downloading method for proxy servers using variable length blocks. The block size is determined dynamically according to the network condition so that the occupied buffer space shrinks. In this paper, we compare the proposed parallel downloading using variable length blocks to the existing method using fixed length blocks by simulation experiments. As a result, it is confirmed that our proposal reduces the redundant traffic and the buffer size required for proxy systems compared to the existing method without degrading the download time. This proposal which absorbs the difference in performance among servers is one of the key technologies for network assurance.