WordPress and subfolder problem

I installed WordPress on my root directory of my server. So I am able to access my site via this url http://mydomain.com. Inside my root folder, I ahve another directory and it is called “subsite”. In this “subsite” folder I have another website with mainly static html files. So I wold like to access my subsite via this URL, http://mydomain.com/subsite/file.html

But everytime I go to http://mydomain.com/subsite/file.html, I got redirect it to my main WordPress site with a 404 page.

Please help me how to go to my subsite via that URL http://mydomain.com/subsite/file.html?

Thank you