S. de Beauvoir's philosophical essay titled 'The ethics of ambiguity' (1947) has been always overshadowed by her most famous book 'The second sex', a seminal work from feminist point of view. This was unfair (so the author's argumentation), since the essay marks the way to understanding of the key thesis of the ambiguous condition of women from 'The second sex'. The paper aims at explaining the standpoint of ambiguity and the sort of ethics implicated by it. The analysis in its first part shows S. de Beauvoir's rejecting the ethics of virtue, happiness and obligation and her trying to rethink the content of ethics on the basis of the ambiguous concept of failure, which on her opinion leads to an authentic existence. The second part focuses on the description of inauthentic attitudes, which are unveiled by S. de Beauvoir as illusionary forms of justification. The third part suggests, that compared with Sartre and his idea of man as a futile passion (passion inutile) S. de Beauvoir does not defend the idea of groundless existence. She, however, insists on avoiding all preliminary justifications based on civilization, age, or culture.