Abstract: From a refinery's intermediate byproduct composed of a sulfide, oxide or arsenide containing copper and arsenic, a liquid extract containing copper and arsenic at a concentration ratio of not less than 1.0 is obtained. The liquid extract is cooled to a predetermined temperature to achieve solid-liquid phase separation, and the liquid resulting from the phase separation is neutralized to pH 6-7 to isolate copper and arsenic in the form of a composite precipitate. The composite copper-arsenic compound mixture thus obtained has a high purity and low production cost and can be converted through a simple treatment into an effective antiseptic for wood.