Basement floor drain backing up

[ninercub]

I have a split level house and when I run the water in the kitchen sink, it backs up the floor drain in the basement. I know the lines are clear from the house to the street, so why would the basement floor drain backup? How do I fix this?

[joed]

The line between the floor drain and the main sewer is clogged. Both the floor drain and the kitchen sink are on the house side of the clog.