We report a 52-year-old woman who was noted to have elevated alkaline phosphatase (ALP), hypercalcemia (Ca: 11.7 mg/dL), and intact parathyroid hormone (intact PTH: 643.1 pg/mL), and then referred to our hospital with suspected hyperparathyroidism. Ultrasound examination of the neck and magnetic resonance imaging showed a mass region in the posterior aspect of the left lobe of the thyroid, and Tl-Tc subtraction scintigraphy showed Tl uptake at the same location. Based on laboratory and imaging studies, she was diagnosed with primary hyperparathyroidism. The excised parathyroid was a large mass measuring 6.8 x 2.8 x 1.9 cm in diameter and weighing 15.4 g. It was soft, covered with a thin capsule, did not infiltrate the thyroid parenchyma, and showed no evidence of malignant process. Histopathological examination showed that it was clear cell adenoma. There was no evidence of metastasis from the parathyroid tumor in other organs. The post-operative course was excellent, and serum PTH, Ca, and ALP levels returned to normal. Among parathyroid tumors, large adenomas are commonly considered to be more likely malignant, but in this case it was benign despite measuring more than 6 cm in diameter. The histopathological type of the adenoma was clear cell adenoma, a very rare type. We report a clear cell adenoma of the parathyroid gland, which has not been described previously in Japan.