A 66 year old man who had been treated under a diagnosis of depression for 10 years was referred to the Kyoto Prefectural University of Medicine, Kyoto because of general fatigue and appetite loss. The patient was diagnosed as having primary hyperparathyroidism (PHPT) based on the increased parathyroid hormone (PTH) and serum calcium levels. Computed tomography revealed solitary adenoma of parathyroid gland. The resection of this solitary adenoma improved the PTH and serum calcium concentrations to normal ranges, which resulted in an improvement in his depressive state. This case suggests that ionic calcium levels contribute to the mental symptoms associated with PHPT. As PHPT is curable, the possibility of PHPT should be taken into account when patients have depressive symptoms.