Previous studies show that, in a binary choice task, people often choose one object between two objects using a simple heuristic (e.g., recognition, familiarity, or fluency heuristic), and that such a simple strategy is ecologically rational. These studies almost exclusively pay attention to subjective knowledge (i.e., familiarity) about two alternatives. However, we pointed out that familiarity of an object presented in a question sentence might affect people’s inferences. Specifically, we hypothesized that, in a binary choice task, when an object in a question sentence was familiar (unfamiliar) to a decision maker, he or she would choose a more familiar (unfamiliar) object from the two alternatives. We call this heuristic “familiarity-matching.” We examined whether people actually employed familiarity-matching and whether familiarity-matching was an ecologically rational strategy. The results of three experiments generally confirmed usage and ecological rationality of familiarity-matching. Experiment 1 showed that if an object in a question sentence was familiar (unfamiliar) to participants, then they were likely to choose a more familiar (unfamiliar) object from the two alternatives;that is, participants indeed employed familiarity-matching. Experiment 2 showed that when participants felt difficult to make a decision, they were more likely to employ familiarity-matching. Experiment 3 showed that familiarity-matching could be applied in an ecologically rational manner in real-world situations. The results of present study collectively shed light on important cognitive mechanisms involved in inference tasks. We believe that the present findings make a substantial contribution to reveal unsolved human cognitive processes.