The "Werewolf game" is a popular multiplayer game wherein "villagers" try to figure out who is a "werewolf" through conversations. Werewolves usually pretend to be villagers. In this paper, we studied conversations in game logs in order to investigate how werewolves' cooperation contributed to increasing the winning percentage of the werewolves' team. As the number of "whispers" that are utterances via werewolves' private chat may be regarded as a measure of the werewolves' cooperation, we investigated the relation between the number of whispers and the winning percentage. As the result, we observed that the winning percentage of werewolves' team increased by 63 points at most when the number of whispers of at least two werewolves was more than 106.