A conjectural variation model is applied to evaluate the effectiveness of generic milk promotion in Japan. A unique feature is the model allows for simultaneous movement of both the price and quantity without a constant fluid milk premium, considering the degree of imperfect competition in the milk market. The major conclusions are : (a) the elasticity of fluid demand with respect to promotion messages for Japan is relatively large compared to studies in other countries ; (b) the marginal rate of return to milk promotion is high and consequently there are opportunities to increase net revenue through increases in milk promotion expenditures in Japan ; (c) the marginal rate of return is underestimated when a constant-price model is used instead of the conjectural variation model, i.e., the conjectural variation model improves the underestimation of milk promotion efficiency by the constant-price model ; and (d) the more competitive the market is, the smaller the marginal rate of return will be. It is remarkable that the marginal rate of return in a price-taking case is below one, i.e., decreases in the expenditures should be required, It should be recognized that the current high marginal rate of return is partly realized by the current degree of competition.