Apparatus and method for processing substrate

An apparatus and method for processing a substrate can reduce the concentration of processing by-products in a chemical solution. The apparatus includes: a substrate rotating device configured to rotate a disposed substrate in a rotational manner; a chemical solution supply apparatus configured to supply a chemical solution to the substrate; a chemical solution discharge line configured to discharge the chemical solution that has undergone the treatment to the outside; a chemical solution circulation line configured to circulate the chemical solution that has undergone the treatment to the chemical solution supply apparatus; and a discharged chemical solution selection device configured to discharge a chemical solution containing a first amount of treatment by-products to the outside through a chemical solution discharge line, and to circulate a chemical solution containing a second amount of treatment by-products through a chemical solution circulation line, wherein the first number of treatment byproducts i