In this paper we give an axiom system of a logic which we call an approximation logic (AL), whose Lindenbaum-Tarski algebra is a strong Bunge algebra (or simply s-Bunge algebra), and show thatFor every s-Bunge algebra , a quotient algebra by a maximal filter is isomorphic to the simplest nontrivial s-Bunge algebra ;The Lindenbaum algebra of AL is an s-Bunge algebra;AL is complete;AL is decidable.