The purpose of this study is to examine the internal transformation process of men who took longterm parental leave. Semi-structured interviews were conducted for 14 men who took a parental leave of more than one month in a private enterprise. The verbal protocols for each participant were analyzed by Modified Grounded Theory Approach (M-GTA). As a result,67 concepts in 31 categories were created based on the data. The results are summarized as below: (1) Decision Making is promoted by Desire to Take Parental Leave,Positive Thought,Information Gathering,Contact with Men Who Took Parental Leave. (2) After passing through Interest in Work-Life Balance and Re-Exploring Career,Self-Centered Career Awareness transforms into Career Autonomy and Integration of Work and Life. (3) After returning to work,there will be actions such as Efficient Ways of Working,How to Work to Take Care of Trust and Efforts to Work Style Reforms.