A 68-year-old man was hospitalized and examined for renal impairment. A laboratory analysis showed hypercalcemia. Although the serum parathyroid hormone and serum 1-25(OH)2 vitamin D3 levels were not elevated, the serum parathyroid hormone-related peptide (PTHrP) level was increased. Immunoelectrophoresis of the urine and bone marrow aspiration indicated multiple myeloma (MM). He was diagnosed with the coexistence of cast nephropathy and light chain deposition disease by a renal biopsy. Notably, PTHrP expression was detected in the myeloma cells based on immunohistochemistry and in situ hybridization. It is therefore important to examine the PTHrP concentration in MM patients with hypercalcemia.