Pembrolizumab is an immune checkpoint inhibitor (ICI) that targets programmed death-1. Although ICIs have shown efficacy in the treatment of lung cancer, they have also been reported to cause a variety of immune-related adverse events (irAEs). Hepatotoxicity is a known irAEs, but currently, there is not enough information on its pathological characteristics and treatment. We report the case of a 70-year-old man with advanced squamous-cell lung cancer who developed severe grade 4 hepatitis on day 8 after receiving carboplatin, nab-paclitaxel, and pembrolizumab as fourth-line therapy. We treated him with steroid therapy the day after a liver biopsy was performed to investigate his pathological features, which led to a rapid and remarkable improvement. Confirmation of immune-related hepatotoxicity by pathological findings allowed the early tapering and discontinuation of steroid therapy. Performing a liver biopsy and verifying histological characteristics are needed for successful treatment with short-term steroids when drug-induced hepatitis caused by anti-cancer therapy including pembrolizumab is considered.