What is a Mirror Site?
ミラーサイトとは、別の Web サイトのコンテンツとデザインを複製した複製サイトです。
The main and duplicate sites look so much alike that their URLs are the only way to tell them apart. The duplicate site will typically have a different domain name or may be hosted on a subdomain of the main site.
Mirror sites are used to distribute load and traffic across multiple servers. In many cases, they are used to host content on servers closest to the audience. They are legal as long as they are created by the site owner or with the permission of the site owner.