This empirical case study examines the marketing in the growth stage of the Japanese game apps market by the classification of business strategies for the game apps such as the lean startup strategy and the imitation strategy. To conduct this examination, a model to calculate the sales performance of game app superstars is proposed using system dynamics techniques, and the transition of key performance indicators is estimated. An examination of the case study and the simulated results of the model suggest that (1) mass media advertisements through game app TV commercials work well for three months, even though five months after an official service release; (2) under game apps that adopted the imitation strategy, the number of multihoming users reaches approximately half of the new registered users and the average revenue per paid user (ARPPU) is higher than that adopted under the lean startup strategy; (3) the game apps adopted the lean startup strategy or imitation strategy coexist in the stage of growth market.