Provided is a tire vulcanizing method enabling an optimum vulcanization operation for each tire by measuring the temperatures of the inner surface and the outer surface of a tire until the demolding timing without damaging the tire and by determining the vulcanized state of the tire accurately based on the measured temperature data to efficiently produce uniformly vulcanized tires especially under conditions where the temperature of a mold fluctuates. During vulcanization of a tire G, the temperatures of the inner surface and the outer surface at a plurality of principal portions representing the tire G are measured, and the demolding timing is determined according to the equivalent degree of vulcanization indicating the degree of progress in crosslinking reaction calculated based on the temperature data measured.