This study aimed to analyze the oncology "drug lag" (i.e., the delay in time required for the approval of oncology drugs) in Japan compared with that in the United States of America (US) or the European Union (EU) and to identify the factors associated with this lag. Using publicly available information, we collected data on 42 approvals of 30 oncology drugs in Japan, the US, and the EU that included dates of drug development initiation, submission, review, and approval. Lags in each step of the process were then examined and compared among the three regions. We found that median submission and approval lag times between Japan and the US were 20.0 and 29.9 months, respectively, while those between Japan and the EU were 14.9 and 21.3 months, respectively. The median review periods for Japan, the US, and the EU were 14.3, 6.0, and 13.2 months, respectively, and the median lag in initiation of oncology drug development between Japan and the US/EU was 38.9 months. The proportion of approvals for which Japanese Phase I registration trials started after corresponding approvals in the US were 39% compared with 47% for the EU. Multivariate analysis suggests that delays in the initiation of drug development and the extended length of the regulatory review period in Japan may contribute to the longer oncology drug lag observed in Japan compared with that of the US or EU.