When we estimate subjective probability about “It will be fine tomorrow”, there are other possibilities about tomorrow's weather (“cloudy”, “rain”, “snow”). In other words, when we estimate subjective probability about an event (focal event), there are other events (alternative events) that are probable but not the targets of probability judgments. Previous studies suggested that alternative events affect the subjective probability of the focal event. In this study, we proposed a descriptive model of subjective probability that considers the effects of the randomness of alternative events to the subjective probability of focal events. Participants were shown various outcomes of multinomial events, and instructed to estimate the subjective probability of the specific events of the multinomial events. Participants answered subjective probabilities by verbal impression (verbal estimate) or objective percentage (percent estimate). The randomness of the alternative events was defined by the expression used in Rappoport & Budescu (1997). The results of multiple regression analysis indicated that the randomness of alternative events significantly affected the subjective probability of a focal event both in verbal estimate or percent estimate, so our model was supported. Some theoretical suggestions were discussed.