Many students in lower grades who study mathematics with computers have difficulty in inputting formulas by using existing methods. It would be much easier for them if they could input formulas naturally, as they appear in textbooks. This paper describes such an interface program module for use in CAI. This module makes it easy for students to input and edit complex formulas solely by key operations, without using a mouse. The difference between the module and existing mathematical expression editors is that it converts formulas into character strings syntactically. In this way, CAI programs can understand the meanings of the formulas.