SMEs encounter many difficulties in integrating and operating various information systems introduced to build smart factories. The source of this difficulty comes from integrated management of data interface between information systems. This research proposes a data interface middleware that can operate and manage various data interfaces between information systems in an integrated manner. First, this study identifies the types of information systems and operational information needed to build smart factories and analyzes the ways of data interface and requirements suitable for the manufacturing environment of SMEs. Structure and detailed functions of the data interface middleware are designed based on the analysis results. The proposed data interface middleware consists of the function layer, engine layer, and DB layer. The function layer is a set of functions for operating the middleware, and the engine layer comprises core engines for executing the functions. The DB layer manages all information that gathers when the data interface is executed. We applied the proposed middleware to connect data between the existing ERP and newly introduced smart factory package software in SMEs. Application results show that the associated data types are consistent in the two systems, and accuracy of the data parsing process is reliable.