1. A thermal sensation determining apparatus comprising: a skin temperature detecting unit configured to detect a skin temperature of a peripheral site of a subject's body; a variation state determining unit configured to (a) determine a variation state determination result of the skin temperature during a first interval to be a first state if a difference between a maximum value and a minimum value of the skin temperature during the first interval is smaller than a temperature threshold or if the difference between the maximum value and the minimum value is equal to or greater than the temperature threshold and a time difference in detection time between the maximum value and the minimum value is equal to or greater than a first time threshold, (b) determine the variation state determination result to be a second state if the difference between the maximum value and the minimum value is equal to or greater than the temperature threshold and the time difference is smaller than the first time threshold and equal to or greater than a second time threshold which is smaller than the first time threshold, and (c) determine the variation state determination result to be a third state if the difference between the maximum value and the minimum value is equal to or greater than the temperature threshold and the time difference is smaller than the second time threshold; a gradient detecting unit configured to detect a gradient of the skin temperature during a second interval longer than the first interval to (d) determine a gradient detection result to be a first gradient if the gradient of the skin temperature is equal to or greater than a first gradient threshold, (e) determine the gradient detection result to be a second gradient if the gradient of the skin temperature is equal to or greater than a second gradient threshold smaller than the first gradient threshold and is smaller than the first gradient threshold, and (f) determine the gradient detection result to be a third gradient if the gradient of the skin temperature is smaller than the second gradient threshold; and a thermal sensation determining unit configured to determine the subject's thermal sensation using an old thermal sensation determination result, the variation state determination result and the gradient detection result to obtain a new thermal sensation determination result classified into levels ranging from “hot” to “cold” based on a “moderate” temperature.