The fundamental mechanisms underlying the formation of process addictions, such as gambling addiction, gaming addiction, and social networking addiction, are not yet understood, and no effective preventive measures or treatments have been found. These addictions are generally associated with excessive activity of the basal ganglia-dopamine system, but the commonality of what constitutes an "addictive object" is not yet clear. We have developed a new experimental model using mice to address this situation. Specifically, we observed the behavior of mice that were asked to choose between two alternatives: a "predictable alternative," which foretells the presence or absence of a reward, and an "unpredictable alternative," which does not foretell anything about the reward. We observed that the predictable option was preferred when the two options offered equal reward probabilities. This preference was maintained even when only the probability of reward for the predictable option was reduced. Furthermore, we confirmed that administering L-DOPA increases the preference for the predictable option. Conversely, the lesioning of D2 receptor-expressing cells in the striatum decreased the preference for the predictable option. Based on these results, we propose a new hypothesis that process addiction is an overestimation of predictable reward signs, i.e., signs that foretell the presence or absence of reward, based on the basal ganglia dopaminergic system.