Objectives: The aim of this study was to investigate both the time‐to‐onset and the onset‐pattern of drug‐induced blood disorders (DIBD) following the administration of monoclonal antibody agents through the use of the spontaneous adverse reaction reporting system of the Japanese Adverse Drug Event Report （JADER) database.Methods: Data in the JADER database from April 2004 to October 2017 were downloaded from the Pharmaceuticals and Medical Devices Agency website.　The DIBD dataset for monoclonal antibody agents was constructed based on the data for the drug information and adverse drug reactions.　The information for the adverse drug reactions was categorized in accordance with the preferred terms of the Medical Dictionary for Regulatory Activities and included thrombocytopenia, platelet count decreased, neutropenia, neutrophil count decreased, leukopenia, white blood cell count decreased, pancytopenia, anaemia, agranulocytosis, granulocyte count decreased, granulocytopenia, and bone marrow failure.　This dataset was then used to calculate the median onset times for the DIBD and the Weibull distribution parameters.Results: The median onset times of the DIBD for gemtuzumab ozogamicin, cetuximab, ramucirumab, trastuzumab, panitumumab, bevacizumab, infliximab, rituximab, trastuzumab, and ibritumomab tiuxetan (90Y) were 4, 10, 13, 14, 14, 14, 16, 16, 27, and 28 days, respectively.　The Weibull distributions for cetuximab, trastuzumab, bevacizumab, infliximab, and tocilizumab were estimated to fit the early failure type profile, while those for gemtuzumab ozogamicin, ramucirumab, rituximab, and ibritumomab tiuxetan (90Y) were estimated to fit the wear out failure type profile.　The Weibull distributions for panitumumab were estimated to fit the random failure type profile.Conclusions: The results of the present study clarified both the most likely time period and the onset‐pattern of DIBD that can occur in patients after the administration of monoclonal antibody agents.