To confirm the efficacy and safety of rituximab (genetical recombination) intravenously administered to idiopathic membranous nephropathy with nephrotic syndrome.

Inclusion Criteria:

1. Patients who undergo kidney biopsy and are diagnosed as having idiopathic membranous nephropathy prior to the obtainment of informed consent
2. Patients who are diagnosed as having nephrotic syndrome prior to the obtainment of informed consent and receive no steroids or immunosuppressants within 12 weeks prior to the obtainment of informed consent
3. Patients with urine protein-creatinine ratio ≥ 3.5 g/gCr at the screening
4. Patients with hypoalbuminemia (serum albumin ≤ 3.0 g/dL) at the screening
5. Patients aged 15 years or older at informed consent
6. Patients who give voluntary written consent after having received adequate information on this study (legally acceptable representatives should also give consent for underage patients, and informed assent should be obtained from children)

Exclusion Criteria:

1. Patients with primary nephrotic syndrome other than membranous nephropathy (IgA nephropathy, minimal change disease, focal segmental glomerulosclerosis and so forth), and patients with secondary nephrotic syndrome (autoimmune disease, metabolic disease, infection, allergic/hypersensitive disease, tumor, and drug-induced disease)
2. Patients with the renal function lowered (eGFR <30 mL/min/1.73 m2 based on CKD-EPIcr formula) at the screening
3. Patients who have used anti-CD20 antibody including rituximab (genetical recombination) prior to the informed consent for idiopathic membranous nephropathy
4. Patients who have participated in another clinical study within 12 weeks prior to the informed consent (enrollment is allowed for those participating in a clinical study in the range of 'Indications' or 'Dosage and Administration' in Japan) or patients who are participating in another study
5. Patients with history of renal transplant
6. Patients with poorly controlled diabetes (HbA1c of 8.0% or higher)
7. Patients who have or are suspected to have active infection (infection requiring treatment with systemic antimicrobial, antifungal, or antiviral agents) at the time of informed consent
8. Patients tested positive for HBs antigen, HBs antibody, HBc antibody, and/or HCV antibody (patients with positive HBs antibody and/or HBc antibody can be enrolled only when HBV-DNA test is negative [less than the detection limit]), or patients with positive HIV antibody or HTLV-1 antibody at the time of the screening
9. Patients with leukopenia (less than 2,000 /mm3), neutropenia (less than 1,000 /mm3), or lymphopenia (less than 500 /mm3) at the time of the screening
10. Patients with history of serious hypersensitivity or anaphylactic reaction to one of the ingredients in the investigational drug or murine protein-containing products
11. Patients who are judged to be life-threatening nephrotic syndrome by the investigator or a subinvestigator
12. Patients with serious comorbidity (e.g., hepatic, renal (excluding idiopathic membranous nephropathy with nephrotic syndrome), cardiac, lung, hematologic, or brain disease)
13. Female patients who are pregnant, lactating, or potentially pregnant, patients who are not willing to use contraceptive measures during the study period, or female patients not willing to use contraceptive measures until 12 months after the last dose of study drug (except for female patients who are unale to pregnant)
14. Patients who are judged to be unsuitable by the investigator or a subinvestigator

If the principal investigator, clinical trial office, main stakeholder conclude that secondary use of individual data obtained in this clinical trial is beneficial for additional analysis, the secondary use of data excluding personal information will be acceptable after publication of results.