Oil adsorbent layers 4 and packed layers 5 are alternately laminated inside a treatment vessel 2, the oil adsorbent layers 4 comprise ten laminated flat sheets 4a composed of polypropylene, and gelled aluminum hydroxide 15 is evenly applied over the surface of each of these sheets 4a on the laminated side. Skills may be needed to replace agents that are reassigned or that are no longer agents of the contact center. For example, a technique may comprise the following steps.