The purpose of this study is to evaluate the efficacy, safety, and pharmacokinetics of ME-401 in the treatment of Japanese participants with Relapsed or Refractory indolent B-Cell Non-Hodgkin's Lymphoma.

Inclusion Criteria:

• Patients aged 20 years or older at the submission of the written informed consent form
• Patients that have undergone therapy after at least 2 prior systemic therapies (anti-CD20 Ab, chemo, and so on) for relapsed or refractory B-cell NHL
• Patients who have not undergone phosphatidylinositol 3-kinase (PI3K) inhibitors to date
• Patients who have not undergone Bruton's tyrosine kinase (BTK) inhibitors to date
• Patients with Eastern Cooperative Oncology Group Performance status (ECOG PS) "0 or 1"

Exclusion Criteria:

• Patients with relapsed or refractory B-cell NHL who is categorized into Small lymphocytic lymphoma (SLL), Waldenström's macroglobulinemia (WM), Lymphoplasmacytic lymphoma (LPL) by WHO classification
• Patients who have been histologically confirmed FL Grade 3b transformation from Follicular lymphoma (FL) to an aggressive lymphoma at least once
• Patients with lymphomatous involvement of the central nervous system
• Patients with uncontrolled clinically significant illness
• Patients with active interstitial lung disease or a history thereof