Japanese has four types of conditional sentences: “Nara-type”, “Reba-type”, “Tara-type” and the natural consequence expression using “To (And-type).” Among these four types, only And-type conditionals cannot express counterfactuals. Masuoka (1993) says that the Japanese conjunction “To (And)” is an index which calculates the relevance between elements. This paper explains this phenomenon based on Relevance Theory (Sperber et al. 1986). Cognitive relevance between the information A and B is defined by correlation coefficient (CR1) or by Loose Symmetry Model (CR2, Shinohara and Nakano, 2007). (1) CR1=P(AB)/P(A)−k∙P(aB)/P(a) (2) CR2=(P(AB)+P(Ab)∙P(ab)/P(b))/(P(AB)+P(Ab)+P(AB)∙P(aB)/P(B)+P(Ab)∙P(ab)/P(b)) In the expression of counterfactuals, the relation P(a)=P(ab)+P(ab)=0 is trivial. Thefore, CR1 is computable only when the coefficient “k” which means the frame range of anti-factual situation equals zero. This computation leads the result of impossibility of And-type counterfactuals. On the other hand, CR2 cannot explain this impossibility because CR2 is always infinite. Therefore, CR1 is the better definition than CR2.