That's basically the time the PHP interpreter takes before sending out output in your case.
Well, the usual fix is caching -- some people have WP sites that more or less act like static HTML sites as far as the public view side is concerned.
Thank you Winteresise your reply. Yes, you are right. It cases php. I already use W3total (already enabled its gzip). After I enabled Apache Gzip via WHM, my FBT reduce in considerable amount. Also page loading time improved. Now first byte time less than 250ms.