The indications for antineoplastic agents are limited in Japan compared with those in the United States. This may be because it takes longer to add indications and the number of additions of indications is lower in Japan than in the United States. To clarify the differences in the timing and number of additions of indications for antineoplastic agents, the agents approved from 2001 to 2020 and sold as of the end of 2020 in Japan and the United States were identified and their additions of indications were compared. Of the 81 antineoplastic agents analyzed, the proportion of agents with additional indications was 71.6 and 63.0%, and the number of additions of indications (median/average per agent) was 2/3.52 and 1/2.43, for the United States and Japan, respectively. The median date of approval for addition of indications was August 10, 2017 and July 3, 2018 for the United States and Japan (p = 0.015), indicating that the indications were added earlier in the United States. The proportion of priority review and orphan drug designation for addition of indications was lower in Japan (55.6 and 34.7%) than in the United States (80.9 and 57.8%) (p < 0.001). When indications were developed with global clinical trials or designated as orphan drugs in the United States, delays in application and approval in Japan against the United States were small (p < 0.020). New indications for antineoplastic agents should be added promptly for Japanese patients because malignancy is the leading cause of death in Japan.