• Reduction in average pain severity as measured by an 11-point Likert scale [ Time Frame: 12 weeks ] [ Designated as safety issue: Yes ]
  

• Pain severity for worst pain and night pain as measured by an 11-point Likert scale. [ Time Frame: 3 months ] [ Designated as safety issue: Yes ]
  
• Patient Global Impression of Improvement scale to measure the degree of improvement at the time of assessment. [ Time Frame: 3 months ] [ Designated as safety issue: Yes ]
  
• Brief Pain Inventory to measure the severity of pain [ Time Frame: 3 months ] [ Designated as safety issue: Yes ]
  
• Beck Depression Inventory-II (BDI-II) total score. [ Time Frame: 3 months ] [ Designated as safety issue: No ]
  
• Safety [ Time Frame: 3.5 months ] [ Designated as safety issue: Yes ]
  

Inclusion Criteria:

• Male or female outpatients aged 20 years or older but less than 80 years at the time of consent.
• Patients with pain due to bilateral peripheral neuropathy induced by type 1 or 2 diabetes mellitus. The pain must have been present for at least 6 months and be evaluable in feet, legs, or hands.
• Patients with HbA1c less than or equal to 9.0 percent at Visit 1.
• Patients in whom HbA1c had been measured 42-70 days before Visit 1 and subsequent HbA1c levels have been within +/- 1.0 percent of the level at Visit 1.
• Patients with a mean of the 24-hour average pain severity scores (round off to a whole number) of 4 or higher, as calculated from the patient diary for 7 days immediately before Visit 2

Exclusion Criteria:

• Patients who have undergone renal transplant or who are currently on renal dialysis.
• Patients who have a history requiring pharmacotherapy within the past year or current history of psychiatric disease, such as mania, bipolar disorder, depression, anxiety disorder, or eating disorder.
• Patients with hypertension with poor control of blood pressure (systolic blood pressure greater than or equal to 180 mmHg or diastolic blood pressure greater than or equal to 110 mmHg)
• Patients with ALT or AST greater than or equal to 100 U/L at Visit 1.
• Patients with use of any of the prohibited concomitant drugs or prohibited concomitant therapies after Visit 1.