In a heat-flux measuring method for measuring an ion flux of plasma generated in a substrate processing chamber using a heat flux, a heat-flux measuring member is exposed to the plasma and irradiated with a low coherent light. The heat-flux measuring member has a three-layered structure in which a first length and a second length of optical paths of the low-coherent light in the first layer and the third layer are measured using optical interference of reflected lights from the heat-flux measuring member. Current temperatures of the first layer and the third layer are obtained based on the measured first length, the measured second length, and data representing thermal-optical path length relationship. A heat flux flowing through the heat-flux measuring member is calculated based on the obtained temperatures, and a thickness and a thermal conductivity of the second layer.