During the COVID-19 pandemic, excess deaths including cancer have become a concern in Japan, which has a rapidly aging population. Thus, this study aimed to evaluate how age-adjusted mortality rates (AMRs) for different types of cancer in Japan changed during the COVID-19 pandemic (2020-2022). Official statistics from Japan were used to compare observed annual and monthly AMRs with predicted rates based on pre-pandemic (2010-2019) figures using logistic regression analysis. No significant excess mortality was observed during the first year of the pandemic (2020). However, some excess cancer mortalities were observed in 2021 after mass vaccination with the first and second vaccine doses, and significant excess mortalities were observed for all cancers and some specific types of cancer (including ovarian cancer, leukemia, prostate cancer, lip/oral/pharyngeal cancer, pancreatic cancer, and breast cancer) after mass vaccination with the third dose in 2022. AMRs for the four cancers with the most deaths (lung, colorectal, stomach, and liver) showed a decreasing trend until the first year of the pandemic in 2020, but the rate of decrease slowed in 2021 and 2022. This study discusses possible explanations for these increases in age-adjusted cancer mortality rates.