Porocarcinoma is a malignancy of the eccrine sweat gland. It may arise de novo or develop from pre-existing poroma and hidroacanthoma simplex. A 56-year-old woman was presented with a pinkish plaque on her left posterior thigh. She was diagnosed with hidroacanthoma simplex 6 years ago, and the same lesion had changed in size and shape. Histopathology revealed intraepidermal proliferation of small-sized basaloid cells, continuing to the proliferation of large polygonal tumor cells. This malignant transformation was consistent with the change in p53 expression pattern. The patient underwent wide excision with clear resection margin, and no evidence of recurrence was noted. We highlight this case in that the transition from hidroacanthoma simplex to porocarcinoma in situ was confirmed in the same tissue, as demonstrated by immunohistochemical staining.