The purpose of this study is to analyze contributions of mortality change by age group and selected causes of death to the increase in life expectancy at birth from 1950 to 2000 in Japan, which has the longest longevity in the world. Using mortality data from Japanese vital statistics from 1950 to 2000, we analyzed contributions of mortality change by age group and selected causes of death to the increase in life expectancy at birth by the method of decomposition of changes and calculated age-adjusted death rates for selected causes of death. Gastroenteritis, tuberculosis and pneumonia largely contributed to an increase in life expectancy in childhood and in the young in the 1950s and 1960s. The largest contributing disease changed from tuberculosis and pneumonia in earlier decades to cerebrovascular diseases in the 1970s. The largest contributing age group also shifted to older age groups. Age-adjusted death rate for cerebrovascular diseases in 2000 was one fifth of the 1965 level. Cerebrovascular diseases contributed to an increase in life expectancy at birth of 2.9 years in males and 3.1 years in females from 1970 to 2000. In the 1990s, the largest contributing age group, both among males and among females, was the 75-84 age group. Of the selected causes of death, heart diseases other than ischemic heart disease became the largest contributor to the increase in life expectancy at birth. Unlike cerebrovascular diseases, cancer and ischemic heart disease contributed little to change in life expectancy at birth over the past 50 years. In conclusion, although mortality from ischemic heart disease has not increased since 1970 and remained low compared with levels in western countries, mortality from cerebrovascular diseases has dramatically decreased since the mid-1960s in Japan. This gave Japan the longest life expectancy at birth in the world. It is necessary to study future trends in life expectancy at birth in Japan.