The article presents the benefits and difficulties of the analysis life writing, especially the diary, by Genetic Criticism (critique genetique). Various Concepts from that methodological field, coined by Philippe Lejeune and Paweł Rodak, are used in the analysis. The text outlines and explains Jan Józef Szczepański’s Diary in this context as well. By using the category of „everyday writing practice” (taken from the Lejeune and Rodak) to define the journal, I shall compare the manuscript of Szczepański’s Diary with the published version, focusing in particular on the former’s special value for interpretation as well as on some issues which disappear in the printed version.