The primary purpose of this study is to analyze the relationship between earnings management and the informativeness of reported earnings. Assuming a case in which the manager observes the firm's pre-managed accounting earnings along with additional private information, we analyzed the effects of the quality of each type of information on the relationship between the degree of income smoothing and the informativeness of reported earnings. Utilizing the framework of Kim & Suh (2008), this study derived the following results. First, we proved that earnings management in equilibrium takes the form of income smoothing. Through this result, we extend the work of Kim & Suh (2008) to research on income smoothing. Second, we showed that the quality of the firm's pre-managed accounting earnings and the quality of the private information additionally observed by the manager have opposing effects on the degree of income smoothing. In equilibrium, as the quality of pre-managed accounting earnings increases, the degree of income smoothing decreases, whereas as the quality of private information increases, the degree of income smoothing increases. Third, we showed that depending on the quality of the two types of information, the relationship between the degree of income smoothing and the informativeness of reported earnings yields different equilibria. If the quality of the private information additionally observed by the manager is held constant, an increase in the degree of income smoothing having a positive effect on the informativeness of reported earnings constitutes an equilibrium. Conversely, if the quality of pre-managed accounting earnings is held constant, an increase in the degree of income smoothing having a positive effect on the informativeness of reported earnings constitutes an equilibrium. The above results imply that the income smoothing phenomenon that occurs when the quality of private information additionally observed by the manager is high is a phenomenon accompanying the manager's decision to enhance the informativeness of reported accounting earnings.