Floatation separator and floatation separation method

A floatation separator which comprises a floatation separation tank for storing therein a raw liquid mixture to be treated which contains floating foreign matters, a skimmer which rotates to scrape upforeign matters floating on the surface of the liquid in the floatation separation tank, and a scum box which has an opening around the liquid surface and is for discharging the foreign matters scraped up with the skimmer from the liquid surface. The floatation separator is further equipped with a water-level switch which lowers the water level of the raw liquid mixture when the rotating skimmergoes apart from the scum box along the rotating direction and elevates the water level of the raw liquid mixture when the skimmer approaches the scum box along the rotating direction. Due to this, anexcess outflow of the raw liquid mixture is prevented and foreign matters are prevented from adhering to the dam of the scum box.