Heat sensing is necessary for human health care and industrial fields. In many cases, the abnormality of human body or machine is appeared as a heat source. For example, a cancer cell is higher temperature by up to 2 or 3 degree on the skin surface over the tumor. Therefore, abnormal detection is possible to sense through the heat distribution in the material. There are many sensors for observing temperature such as thermocouple, thermistor, thermography, and so on. However, sensing in deep layer of material using these sensors are difficult. This paper proposed the sensing method for detecting heat source inside the material by using heat flow. The method uses an algorithm that heat flow is changed depending on temperature difference. Some experiments are conducted to verify the proposed method.