A practical method for enzymatically synthesizing c-di-GMP with excellent productivity is provided. A diguanylate cyclase having physical and chemical characteristics (A) to (F): (A) catalytic action on reaction “2 GTP→c-di-GMP”; (B) a molecular weight of 19800±2000; (C) an optimum pH of 7.3 to 9.4; (D) an optimum temperature of 35 to 60° C.; (E) thermal stability as the remaining activity of 90% or higher after heated for 60 minutes under conditions of 50° C. and pH7.8; and (F) the presence of GGDEF (SEQ ID NO:26) domain and the lack of amino acid sequence KXXD (SEQ ID NO:23) in the i-site.