I study a model of strategic disclosure of a private signal to a rival in the presence of a payoff externality. In the model, two agents forecast the unknown true state of a future period. Specifically, the quality of the signal is also private information. Hence, which quality of signal is revealed for which incentive is the main question of interest. I show that, even when disclosure is costly, the revealing equilibrium, where an agent voluntarily reveals his signal, can exist and it should be a monotone equilibrium. Asymmetry between the penalty for an incorrect forecast and the reward for a correct forecast is a necessary condition for the existence of this revealing equilibrium. If the penalty is larger than the reward, the unique revealing equilibrium is a separating equilibrium, where only the low quality signal is disclosed in order to induce the rival's imitation. On the other hand, if the reward is much larger than the penalty, the unique revealing equilibrium is a pooling equilibrium, where the signal is always revealed in order to induce the rival's deviation. If the reward is not much larger than the penalty, no revealing equilibrium is robust to a costly disclosure. (JEL D81, D82)