A tire vulcanizing method reduces consumption energy and increases productivity of vulcanization. This method involves supplying an inert gas to an internal space of the tire via a supply pipe, discharging the gas from the internal space via a return pipe, and supplying the gas from the return pipe to the supply pipe by a rotation type circulation device. A heating device heats the circulated gas. An internal outlet gas temperature sensor detects a temperature of the gas in the return pipe. A controller decreases a rotating speed of the circulation device when the return pipe gas temperature is a rotating speed decrease temperature or more.