A 55-year-old woman is described whose serum calcium and phosphorus levels had to be controlled by the administration of calcium salts due to the absence of normal parathormore secretion after the removal of her thyroid gland. After the operation, the serum calcium levels were well controlled by the administration of calcium carbonate and alfacalcidol. However, after the added administration of famotidine, the serum calcium level decreased within a few days, and the appearance of tetania was observed. As a result, the daily dosage of calcium carbonate and alfacalsidol were increased, however the serum calcium level still did not improve. When the administration of famotidine was stopped, however, the tetania was disappeared and the serum calcium level increased. The findings of this case suggest that the calcium carbonate became insoluble in water due to an increase in the intragastric pH level caused by the addition of famotidine, which led to a decrease in calcium absorption from the small intestine.