1. A method for converting a formula in a source code to a human language sentence, the method, executed by a processing unit, comprising: extracting a first variable included in a formula from the source code described in extensible business reporting language (XBRL), the formula having at least two variables which are divided into a left side and a right side by an operator; assigning a predetermined operator to any variables in the formula not provided with a specific operator; searching the source code for a first XBRL element that defines the first variable; identifying a first label name corresponding to the XBRL element by searching a table that defines label names for XBRL elements, respectively; extracting a second variable included in the formula but in a different side of the formula from the first variable; searching the source code for a second XBRL element that defines the second variable; identifying a second label name corresponding to the second XBRL element by searching the table; extracting the operator included in the formula from the source code; and replacing the operator with a character string indicating a function of the operator, wherein the character string, the first label name, and the second label name create a human language sentence.