Inclusion Criteria:

          -  Has a histopathologically documented IDH1 mutated WHO grade II glioma according to the
             2016 WHO classification.

          -  Has confirmed IDH1 mutation at the R132 locus by testing at the central laboratory
             conducted during the screening period.

          -  Has no prior anticancer treatment (including chemotherapy and radiotherapy) for glioma
             except craniotomy or biopsy.

          -  Has at least 1 measurable and non-enhancing lesion.

          -  Has an interval of at least 90 days from the latest surgery.

          -  Has no sign of malignant transformation including the appearance of enhancing lesions
             and/or rapid growth of non-enhancing lesions.

          -  Has an Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 to 1.

        Exclusion Criteria:

          -  Has had a histopathological diagnosis of WHO grade III or IV glioma.

          -  Has had a contrast enhancing lesion on brain MRI.

          -  Has received a prior treatment with any mutant IDH1 inhibitor.

          -  Has received other investigational products within 28 days before the start of the
             study drug treatment.

          -  Has an active infection requiring systemic treatment.

          -  Has multiple primary malignancies.

          -  Has a history of clinically significant cardiac disease.

          -  Is a pregnant or lactating woman.