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 determine a variation state of the skin temperature during a first interval to be a first state in which the variation in skin temperature is the most moderate, a second state in which the variation is more significant than that in the first state, or a third state in which the variation is more significant than that in the second state, to obtain a variation state determination result; a gradient detecting unit configured to detect a gradient of the skin temperature during a second interval to determine the gradient to be a first gradient which is the greatest, a second gradient which is smaller than the first gradient, and a third gradient which is smaller than the second gradient, to obtain a gradient detection result; and a thermal sensation determining unit configured to determine the subject's thermal sensation using the variation state determination result and the gradient detection result to obtain a thermal sensation determination result classified into levels ranging from “hot” to “cold” based on a “moderate” temperature.