We report a case with bipolar II disorder having mixed features, in which refractory insomnia persisted. We diagnosed his case as mixed depression with mood fluctuations because increased impulsivity and buying sprees became remarkable, with diminished ability to think or concentrate. Switching to carbamazepine and risperidone improved his mood fluctuations and impulsivity. Nevertheless, his intermittent awakening (fragmentation of the sleep-wake rhythm), related dysfunctional beliefs, anxiety about sleep, and mild impulsivity persisted. The addition of various benzodiazepine sleeping drugs, bromovalerylurea, and antipsychotics did not improve insomnia. His intractable insomnia was markedly responsive to gabapentin, engendering further improvement of mood symptoms. Eventually, its efficacy achieved his reinstatement at work. Results of this case suggest the clinical use of gabapentin for treating bipolar disorder, especially in cases with intractable insomnia, which is a very important point in the symptoms and therapeutics of bipolar disorder.