Abstract: The present invention provides a method for measuring the gastric acidity of a mammal using a 13C-labeled carbonate compound. Specifically, the present invention relates to a method for measuring the gastric acidity of a mammal including the following steps: (1) using, as a test sample, expired air of a mammalian subject excreted at any point in time within 30 minutes after oral administration of a predetermined amount of a 13C-labeled carbonate compound, measuring behavior of 13CO2 in the expired air; (2) comparing the behavior of 13CO2 (measured 13CO2 behavior) obtained in step (1) with the behavior of corresponding 13CO2 (reference 13CO2 behavior) that has been obtained beforehand in a control mammal; and (3) determining the gastric acidity of the mammalian subject based on a difference between the reference 13CO2 behavior and the measured 13CO2 behavior obtained above.