The present invention provides a temperature sensor and a method for measuring a temperature, capable of measuring in a high response speed accurately. The temperature sensor of the invention in accordance with a time-of-life of a fluorescent light that is produced by a fluorescent material (21), and is provided with a light source (12) for imaging an excitation light, a fluorescent material (21)for producing fluorescent light by the excitation light, a photoreceptor element (13) for detecting the fluorescent light and outputting a fluorescent light signal in accordance with the fluorescent light intensity, and a signal processing circuit (14) for calculating a temperature based on the fluorescent light signal from the photoreceptor element, where the signal processing circuit (14) calculates integral values for the fluorescent light signals in the respective intervals A through C that have identical time durations, and then, based on the integral values, for two intervals, calculates a difference value, and then calculates a ratio of two difference values, to be a variable that varies in accordance with the temperature, and then references the relationship between the temperature and the ratio between difference values, stored in advance, to convert the calculated variable into the temperature.