Provided is a refractory thickness measuring method comprising irradiating a tubing material with a radiation, detecting an attenuated radiation having passed through the tubing material and a refractory on the inner side of the tubing material, detecting the irradiation-position surface temperature of the surface of the tubing material irradiated with the radiation and the detection-position surface temperature of the surface of the tubing material where the attenuated radiation is detected, calculating the attenuated intensity of the refractory by eliminating the attenuated intensity of thetubing material from the attenuated intensity of the attenuated radiation detected, calculating a refractory thickness from the attenuated intensity of the refractory, and calculating the irradiation-side refractory thickness and the detection-side refractory thickness from the refractory thickness, by using the irradiation-position surface temperature and the detection-position surface temperature.