In this Japanese court decision, Spec Computer is sued (this is the appeal) by game manufacturer Konami for copyright infringement. Spec Computer loses and is forced to pay Konami. Konami had created a game that simulated a love story, where the player/main character would progress over a specific set of days, building personal "stats" and romantic relationships. Spec created a memory unit that could boost a player's stats and start the game at any point in the game's calendar. Konami argued, and the court agreed, that this action changed the essential contents and purpose of how the game was meant to be enjoyed, thus infringing upon the author's "right to preserve the integrity of a work." The case is similar to the Nintendo of America v. Lewis Galoob Toys case in the US, where Nintendo challenged Galoob's right to produce the Game Genie (which did basically the same thing as Spec Computer's product); Nintendo lost. Yet in Japan, the original game creator won out over the party who created the means to modify it. When comparing these two cases, it seems as if Japanese copyright law is enforced more strictly than American.
As other sources indicate, Japanese anime and manga artists happily live with fan-made comics starring copyrighted characters (called dojinshi, or doujinshi). In fact, much of the talent and creativity within the industry can be attributed to artists starting out in the dojinshi field. But in Konami v. Spec Computer it is clear that such infringement is technically against Japanese law as much as it would be against American law (even more so, given this case's similarity to Nintendo v. Galoob). So it is not the law that dictates the products of Japanese fan-culture, but rather common opinion and recognition of the positive effects to come out of certain forms of infringement.