This research investigated developmental changes in self-regulation, which is divided into two categories: self-assertion and self-inhibition, in comparison of four periods i.e., infancy, childhood, adolescence and youth. Japanese university students (N=132), 21 years old, evaluated their self-assertive behavior and self-inhibitive behavior in each of the periods. The results were as follows: (1) The scores of self-evaluation of self-inhibition gradually increases from infancy to youth. (2) As to self-evaluation of self-assertion, the score in childhood is the highest and that in adolescence is the lowest. (3) Each score of all four periods shows significant correlations with its closest periods. (4) As to the scores of self-evaluation of either self-assertion or self-inhibition, there is no significant relation between the period of infancy and that of youth.