The original form of the Sinc approximation is efficient for functions whose boundary values are zero, but not for other functions. The typical way to treat general boundary values is to introduce auxiliary basis functions, and in fact such an approach has been taken commonly in the literature. However, the approximation formula in each research is not exactly the same, and still other formulas can be derived as variants of existing formulas. The purpose of this paper is to sum up those existing formulas and new ones, and to give explicit proofs of those convergence theorems.