Researchers have studied trends in the inequality of educational opportunity by measuring social origin through parental socioeconomic positions. However, recent studies expect that the influences of grandparents’ socioeconomic resources may have increased, suggesting the parent-child association is becoming not to reflect the influence of parents per se over time. To examine this, we demonstrate how associations between parents’ and grandparents’ educational attainment and their (grand)child’s educational attainment has changed across cohorts by utilizing three-generation data from multiple nationally representative social surveys in Japan. The results reveal that the influence of grandparents’ educational attainment has increased over cohorts for grandsons, indicating that the declining influences of parents’ educational attainment will be underestimated without introducing the grandparental generation. No increasing trends in grandparent-granddaughter association of educational attainment are not found. Incorporating other familial members into the measurement of social origin allows us to disentangle the (un)changes in parent-child intergenerational association over time.