We have developed a novel market game, Prediction Game and Experience Sharing (PreGES, pronounced PreGuess), that harnesses people's collective prediction and experience sharing to forecast success or failure of new items (e.g., products, services, UI designs). Companies can register their new items on this market (as a testbed) to ask for collective opinions. In each PreGES trial session, participants makes their own best predictions on other people's overall opinions about the new items to get incentives (e.g., real opportunities to experience the items) and have fun in gambling-like games. As a participant�s guess (or portfolio) approaches the collective guess of all participants, he or she has a greater chance of winning an incentive. Participants improve the accuracy of their next prediction by sharing experiences. As participants have more trial sessions, their collective prediction converges into one common opinion (forecasting the success or failure of new items).