The purpose of this study is to explore the internal transformation processes about working and becoming a parent of full-time employed mothers during parental leave for their first child. Semistructured interviews were conducted with 15 mothers, and the verbal protocols were qualitatively analyzed according to the Modified Grounded Theory Approach (M-GTA). As a result, a hypothesizedmodel consisting of 11 categories, 9 subcategories, and 48 concepts was formed. The major findingswere as follows: (1) Mothers who decide to continue working after childbirth view parental leave not only as a termfor childrearing, but also as an opportunity for career development, and engage in various on-lineself-enrichment activities. (2) Immediately after childbirth, mothers are immersed in their role and cannot think about career.Psychosocial development is seen in the process of being a parent. (3) Influenced by the parental role’s responsibility and co-parenting with the husband, mothers became more conscious of themselves as members of the family, and aim for a work style that prioritizes time spent with family. (4) They feel the relative emotions of anticipation and anxiety about their multiple-role life after returning to work, and the ambivalent emotions of anticipation and loneliness of leaving their children at the nursery. It is hoped that these results will help to deepen our understanding of the psychology of mothers on parental leave and provide material for considering support for mothers on parental leave.