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Abstract A 72‐year‐old man, diagnosed with advanced lung squamous cell carcinoma, was administered of cisplatin plus gemcitabine with necitumumab, a human monoclonal antibody that binds to the epidermal growth factor receptor (EGFR), as a sixth‐line treatment. Tumor shrinkage was observed, but asymptomatic grade 4 hypomagnesemia occurred on day 8 of the second cycle. He received magnesium replenishment and hypomagnesemia recovered on day 40, but tumor progression was observed during the period of magnesium correction. Hypomagnesemia is known as a major adverse event of treatment with anti‐EGFR antibodies, but there have been no case reports of severe hypomagnesemia or its clinical course.