Six days after the onset of influenza B symptoms, a 14-year-old Japanese boy presented with encephalopathy-like symptoms, somnolence, irritability, and childishness, which we first considered was an atypical type of influenza-associated encephalopathy because the infection symptoms disappeared by day 4. His encephalopathy-like symptoms gradually improved, although he had repetitive hypersomnia attacks. Owing to the patient's clinical presentation and normal interleukin-6 levels in the cerebrospinal fluid during the first period of hypersomnia, we diagnosed him with Kleine-Levin syndrome (KLS) triggered by influenza B. The preceding influenza infection was not only a diagnostic clue of KLS but also a diagnostic confounding factor.