In comparison with the other developed countries, bicycle is widely used for transportation in all ages in Japan. In 2006, the number of death caused by bicycle related injury in Japan was worst among the member of Organisation for Economic Co-operation and Development (OECD). The aim of this study is to clarify the features of bicycle related severe head injury in Japan. One thousand and one hundred one cases had been registered in the Japan Neurotrauma Data Bank from 2004 to 2005. We enrolled 580 cases (52.7%) of head injury by traffic accident. These cases were divided to Bicycle related head injury group (BR group) and the others (non-BR group). The number of BR group was 149 cases (25.7%). Only 2 infants (0.7%) had put on helmet in BR group. The female ratio and the mean age of BR group were higher than that of non-BR group. The proportion over 65 years of age in BR group was higher (33.6%) than that of non-BR group (22.7%). The average of Glasgow Coma Scale (GCS) score in BR group is higher in comparison to non-BR group. The number of cases with lucid interval was higher as compared to non-BR group. Though the rate of the case with focal brain injury was high and the average of Injury Severity Score (ISS) was low in BR group, there was no statistical significance in the mortality rate among the two groups. This discrepancy was caused by elderly patients of BR group. And then there is a big number of elderly patients with bicycle related injury in Japan. This might have been caused by the rapid increase in aging society and decrease in birth rate. The results of our study provide the important information for taking preventive countermeasures against bicycle related head injury such as safety education, helmet and other campaigns not only for youngsters but also for elderly cyclist in Japan.