Inclusion Criteria:

          -  Participants with histological diagnosis of B-cell non-Hodgkin's lymphoma (NHL) as
             follows:

               -  Cohort 1: Follicular lymphoma (FL)

               -  Cohort 2: Diffuse large B-cell lymphoma (including primary mediastinal B-cell
                  lymphoma and transformed FL)

          -  Participants who have confirmed EZH2 gene mutation of tumor in central laboratory

          -  Participants who have measurable disease

          -  Participants who had previous therapy with systemic chemotherapy and/or antibody
             therapy and for which no standard therapy exists

          -  Participants who had progressive disease or did not have response (complete response
             or partial response) in previous systemic therapy, or relapsed or progressed after
             previous systemic therapy

          -  Participants with Eastern Cooperative Oncology Group performance status of 0 to 1

          -  Participants with life expectancy of ≥3 months from starting study drug administration

          -  Participants with adequate renal, liver, and bone marrow function

          -  Male and female participants ≥20 years of age at the time of informed consent

          -  Participants who has provided written consent to participate in the study

        Exclusion Criteria:

          -  Participants with prior exposure to EZH2 inhibitor

          -  Participants with a history or a presence of central nerves invasion

          -  Participants with malignant pleural effusion, cardiac effusion, or ascites retention

          -  Participants with allogeneic stem cell transplantation

          -  Participants with medical need for the continued use of potent inhibitors of
             Cytochrome P450 3A (CYP3A)or potent inducer of CYP3A (including St. John's wort)

          -  Participants with significant cardiovascular impairment

             · Participants with prolongation of corrected QT interval using Fridericia's formula
             to > 480 milliseconds (msec)

          -  Participants with venous thrombosis or pulmonary embolism within the last 3 months
             before starting study drug

          -  Participants with complications of hepatic cirrhosis, interstitial pneumonia or
             pulmonary fibrosis

          -  Participants with active infection requiring systemic therapy

          -  Women of childbearing potential or man of impregnate potential who don't agree that
             both the participant and his/her partner will use a medically effective method for
             contraception for periods from before informed consent to during the clinical study
             and 30 days later (for males 90 days later) from last administration of study drug

          -  Woman who are pregnant or breastfeeding

          -  Participants who were deemed as inappropriate to participate in the study by the
             investigator or sub-investigator

          -  Have any prior history of T-cell lymphoblastic lymphoma/T-cell acute lymphoblastic
             leukemia or myeloid malignancies, including myelodysplastic syndrome