Tort liability was established, when the Tortfeasor’s behavior caused the victim’s damage. In Principle the plaintiff must prove causation in tort law. A public nuisance often involve complex scientific and technical issues, so it is difficult for the plaintiff (victim) to prove the existence of a causation. In order to reduce the Plaintiff’s burden of proof in cases of public nuisance, Japanese theory has proposed causation theory in epidemiology in cases of the public nuisance. But what is the causation? What is the applicable standard of causation theory in epidemiology ? This article would make a description with the introduction of German and Japanese law practice and theory, as well as organize the opinions of our practice and theory, hoping to provide comparative law aspect to the whole study.