Zero-sum belief is the belief that someone’s gains are another’s losses. Assuming that beneficiaries’ zero-sum beliefs let them perceive benefactors’ cost resulting from giving benefits, this study examined whether the zero-sum belief increases the occurrences of grateful feelings and expression in apologetic form, which is represented by “sumimasen” in Japanese. We manipulated participants’ zero-sum beliefs and rewarded them for the task. Thereafter, we asked participants what they wanted to say, how they felt, and how much they perceived our (i.e., benefactors’) cost. The results revealed that participants whose zero-sum beliefs were experimentally strengthened were inclined to select the grateful expression in apologetic form from some options to convey what they wanted to say, though grateful feelings in apologetic form and perceived costs were not significantly affected. These results suggested the possibility that individuals’ zero-sum beliefs let them express their gratitude in apologetic form independently from the extent to which they have such feelings or perceive benefactors’ cost.