When we examine the theory of fiction, we usually consider the impact on arts and literatures. However, the most famous theorist Kendall Walton has attracted the attention other than arts and literatures. This paper considers an application of the Walton's theory in social philosophy. The contents of this paper are as follows. In Section 2, I will examine theory of make-believe. In Section 3, I will purpose an application to sports of theory of make-believe. Finally, in Section 4, I will consider John Searle’s idea: brute and institutional (or social) fact. And, I suggest that theory of make-believe is beneficial in social philosophy.