We have two manufacturing units. Distance between two is about 15 k.m.
Both units have boardband connection. We want to connect both unit network/server with using boardband/ internet.

What technology we should use for this purpose.

I'll vote for VPN.

You can either use VPN as suggested or remote access software like Logmein (https://secure.logmein.com/home.asp?lang=en) which allows you to access another computer through internet.