We report a case of squamous cell carcinoma arising in giant porokeratosis in an 88-year-old woman. She had a 20 × 12 cm sized erythematous scaly patch with central ulceration on the right parietal area. On histopathologic examination, a skin biopsy specimen from the peripheral ridge of erythematous lesion revealed porokeratosis, and a specimen from the central ulceration showed well differentiated squamous cell carcinoma.