Methods of filtering unwanted traffic in ISPs are categorized into inbound filtering at receiver ISPs and outbound filtering at sender ISPs. Outbound filtering is effective because it can directly utilize sender information such as dynamic IP address information, but not widely used in the current Internet. This is because outbound filtering does not benefit the ISP that installs it and pays the cost, but benefits ISPs that receive unwanted traffic from the installing ISP. The objective of our study is to solve this incentive problem.We propose an incentive mechanism where ISPs pay penalties for sending unwanted traffic to peering ISPs. This payment is combined with payments of transit fees so that the mechanism will be deployed through the current ISP relationship. The key question here is whether the mechanism will be deployed or not, and we confirmed through simulations that the mechanism is spontaneously adopted by a sufficient number of ISPs who behave in a selfish manner. Accordingly, outbound filtering was also widely installed. In our simulation scenario, there are some ISPs who send little unwanted traffic and should not install the outbound filtering due to its cost. Our simulation showed that with an appropriate penalty charge parameter setting, our proposed mechanism can achieve the state that almost all ISPs who should install the outbound filtering install the filtering, and otherwise not.