The purpose of this study was to investigate factors affecting the decisions regarding second births. Especially, analysis focused on the connections between desire for a second birth and the family factors such as family values and marital relations because they have been rarely known until now. Data was from 1,156 married women that had only one child and had been part of the national data collected by KIHASA in 2003. Major findings can be summarized as follows. First, it was found that value-related factors affect the likelihood of a second birth. The degree of parental responsibility is negatively related with the desire for a second birth. However, the perceived ideal number of children is positively related to it. Second, it was found that the quality of family relations influences the decision for the second birth. Particularly, marital satisfaction is likely to be the most important factor tending to mediate the effects of socio-demographic factors and value-related factors on the plan for a second birth. These results suggest that the effects of social norms on the number of children and timing of childbearing which exist in the family are becoming weaker. Nevertheless, the traditional value of children such as a preference for a son can still be found.