The invention provides a treating device for a polarizer manufacturing waste liquid, capable of efficiently recovering a high-quality potassium iodide solution from a polarizer manufacturing waste liquid. The treating device (1) for the polarizer manufacturing waste liquid for recovering the potassium iodide from the polarizer manufacturing waste liquid comprises a first processing device (10) forperforming solid-liquid separation on an educt formed by concentrating and crystallizing a polarizer manufacturing waste liquid and performing and producing a potassium iodide solution containing a boron and a polyvinyl alcohol in a reduced amount of impurities; a first impurity adsorption device (30) for adsorbing polyvinyl alcohol remaining in the obtained potassium iodide solution onto a polyvinyl alcohol adsorbent; and a second impurity adsorption device (40) for adsorbing boron remaining in the potassium iodide solution passing through the first impurity adsorption device (30) onto the boron selective adsorption resin.