Refactoring makes existing source code more understandable and reusable without changing observable behavior. Therefore, applying refactorings to existing source code and reversing the effects of past refactorings are inseparable from tool support to software development and maintenance. This paper proposes a powerful undo mechanism that uses a chain of past refactorings for each source file and determines which refactoring is undoable by monitoring the last refactoring in every chain. The proposed undo mechanism can make an undesired refactoring accurately undone although the refactoring has affected multiple source files. Moreover, it permits a programmer to compatibly execute operations of refactoring undo and editorial undo/redo without the limitation of manual edit of files. A running implementation of the mechanism has been integrated into our developed refactoring browser.