Infection by hepatitis B virus is one of the major health problems of this nation. HBsAg positive rates of general population and school children were known to be as about 8 percent and 3.9 to 5.9 percent respectively. To study the incidence rate of hepatitis B infection in school children of rural area, author had examined 475 school children of relatively isolated agricultural area for baseline prevalence of hepatitis B virus serologic markers and followed up 415 school children during 10 months to determined the frequency of serologic conversion. The major results are summarized as followings: 1) Among the 278 susceptible children who were followed up, 26 had seroconversion for HBsAg or Anti-HBs. Therefore, the cumulative incidence rate during 10 months is estimated 9.4%. 2) The incidence rate of hepatitis B infection tends to increase with age (6-9yrs: 3.2%, 10-14yrs: 9.5%, 15-17yrs: 18.9%), and the incidence rate in male (13.0%) was higher than in female (5.7%). 3) The incidence rates of hepatitis B virus infection were not different statistically among three economic classes (The rates of lower, middle and higher class were 11.8%, 7.1% and 10.5%.). 4) The incidence rates of hepatitis B virus infection were not different statistically between visitors and non-visitors of clinic or hospital, dental clinic, persons received IV and not received IV, and persons with familial history and without familial history of liver diseases. Therefore all of these factors were not identified as risk factor of hepatitis B virus infection. And the transmissibility within the class of school was not recognized, too. 5) Among the 25 children who were HBsAg positive when enrolled, 15 (60%) were still HBsAg positive, who were identified as chronic carrier 15 of 415 school children were chronic carriers, then chronic carrier rate was estimated 3.6%, and there was no difference between sexes. 6) Of 38 children who had been Anti-HBs positive when enrolled, 5 (13.2%) lost Anti-HBs. Therefore, the loss rate of Anti-HBs per year is estimated to be 15.8%.