We developed the Coparental Regulation Inventory to assess the regulatory behavior of the mothers in involving fathers with child rearing. We translated and modified the short form of the Parental Regulation Inventory (PRI) for Japanese couples in different stages of child rearing. An online questionnaire was conducted with mothers (n = 500) and fathers (n = 500) whose youngest child was less than 21-years-old. Exploratory factor analysis identified two factors, which were labeled "encouragement" and "criticism". The resulting Coparental Regulation Inventory (the modified PRI) had high internal consistency and test-retest reliability. The construct validity of the scale was supported by its correlation with parenting alliance, marital satisfaction, and the father's involvement. These findings suggest that the scale is an adequate instrument for identifying the behaviors of mothers related to coparenting. In addition, we examined the frequency of encouragement and criticism used by the mother in relation to the child-rearing stage using cross-sectional analysis. In the mothers' reports, mothers with infants and children encouraged fathers more than mothers with early and late adolescents. Mothers with late adolescents criticized fathers less than mothers with infants. In the fathers' reports, mothers gave more encouragement to fathers who had infants than at any other age, whereas the child's age was not related to mothers' criticism perceived by the fathers.