Risk factors for low birth weight (LBW) have been indicated in many studies, but in Ja pan few studies have examined the amount of reduction in birth weight (BW). The goal of this study was to examine the association between maternal smoking among pregnant women and subsequent reduction in BW, corrected by the effect of potential confounders. To assess the effect of background factors other than maternal smoking, we established a model to estimate the contribution of each explanatory variable using logarithmic multiple regression. We then used the adjusted BW model to evaluate the direct effect of maternal smoking. To obtain information on maternal characteristics, including smoking status and characteristics of their infants, including BW, we conducted a questionnaire survey. After statistical adjustment for background factors, the mean of BW among infants of participants who smoked during pregnancy was roundtable significantly lower than that of non-smoking participants, but there was no verification of a dose-response relationship. However, mean BWs were not significantly different when comparing participants who quit smoking during pregnancy to non-smoking participants, suggesting that stopping smoking during pregnancy is beneficial.