1. A source code equivalence verification device that verifies equivalence between an original source code and a source code after refactoring execution, comprising: a processor; and a storage device that stores instructions that cause the processor to execute: a source code input unit that receives a non-refactored source code and a refactored source code; a refactoring pattern input unit that receives refactoring pattern information of the source code; a source code information generating unit that performs lexical analysis and syntax analysis on each of the non-refactored source code and the refactored source code, converts the non-refactored source code and the refactored source code into structure graphs, and generates non-refactored source code information and refactored source code information; a normalizing unit that normalizes the non-refactored source code information and the refactored source code information with reference to normalization information defined for refactoring pattern information, and generates normalized non-refactored source code information and normalized refactored source code information; a structure comparing unit that compares a structure of the normalized non-refactored source code information with a structure of the normalized refactored source code information; a symbolic execution unit that performs symbolic execution on the normalized non-refactored source code information and the normalized refactored source code information; and an equivalence determining unit that compares symbolic execution results obtained by the symbolic execution unit, and determines equivalence between the non-refactored source code and the refactored source code, wherein the normalizing unit further includes a change position specifying unit that compares the non-refactored source code information with the refactored source code information with reference to change position specifying information defined for the refactoring pattern information, and generates change position information specifying a structural change position of a source code, the symbolic execution unit further includes an abstracting unit that abstracts the normalized non-refactored source code information and the normalized refactored source code information with reference to change position information of the non-refactored source code information and the refactored source code information extracted by the change position specifying unit and abstraction information defined for the refactoring pattern information, and the symbolic execution is performed on abstracted non-refactored source code information and abstracted refactored source code information generated by the abstracting unit.