Background: Tumor-treating fields (TTF) are alternating electric fields applied continuously to the brain by attaching 2-pair arrays on the scalp. Although TTF therapy has demonstrated efficacy against supratentorial glioblastoma (GBM) in adults, its safety and efficacy in children have not been confirmed. In Japan, off-label use of medical devices is almost impossible because the national health insurance system does not cover the cost of off-label use of drugs and medical devices. Therefore, TTF therapy cannot be applied to the treatment of pediatric GBM.[Objectives] The investigator-initiated clinical trial aims to expand regulatory approval of TTF therapy for pediatric GBM treatment based on safety and exploratory efficacy data. Methods: Patients aging between 5 and 17 years with histopathological diagnosis of GBM (either newly diagnosed or first-recurrence), which located in the supratentorial region would be included. All the patients will receive TTF therapy for 28 days per course for up to 26 courses until the end-of-therapy criteria are met. The primary endpoint is the adverse event rate with causality. The secondary endpoints include various time-to-event measures and QoL. In total ten patients will be enrolled. Current Status: Discussions with the Pharmaceuticals and Medical Devices Agency (PMDA) led to a tentative consensus that the accumulated data on the efficacy of NovoTTF-100A for adult GBM may be extrapolatable to pediatric GBM if the trial is able to demonstrate efficacy equivalent to that found in previous, adult studies. On the other hand, the combination of the pediatric safety data gathered in this trial and the findings of international studies, including clinical trials and post-marketing surveillance studies, may expedite approval of the device for pediatric GBM treatment. The trial started patient enrollment in April, 2021 with the supervision of the Advanced Medical Care administration system and is currently awaiting the first eligible patient.