A surface temperature measuring method of measuring a surface temperature of a temperature measurement target material by detecting, by use of a radiation thermometer, thermal radiation light emitted from a surface of the temperature measurement target material in a cooling process with water, the method comprising: providing a radiation thermometer comprising a light receiving unit, and a housing having an opening on a temperature measurement target material side, the housing storing, in an inside of the housing, at least the light receiving unit of the radiation thermometer; interposing an optical glass configured to transmit the thermal radiation light between the temperature measurement target material and the light receiving unit of the radiation thermometer, wherein the optical glass is fit and sealed in the inside of the housing; locating an end surface of the optical glass on a temperature measurement target material side adjacently to the surface of the temperature measurement target material and measuring the surface temperature of the temperature measurement target material; and providing a keeping member configured to keep a gap between the surface of the temperature measurement target material and the end surface of the optical glass on the temperature measurement target material side substantially constant; wherein the light receiving unit of the radiation thermometer receives the thermal radiation light emitted through the water that is present in the space between the surface of the temperature measurement target material and the end surface of the optical glass on the temperature measurement target material side, and wherein the surface of the temperature measurement target material on which the radiation thermometer detects the thermal radiation light is a plane that is substantially vertical to a horizontal direction and wherein the gap between the surface of the temperature measurement target material and the end surface of the optical glass on the temperature measurement target material side is 1.0 mm or shorter. providing a radiation thermometer comprising a light receiving unit, and a housing having an opening on a temperature measurement target material side, the housing storing, in an inside of the housing, at least the light receiving unit of the radiation thermometer; interposing an optical glass configured to transmit the thermal radiation light between the temperature measurement target material and the light receiving unit of the radiation thermometer, wherein the optical glass is fit and sealed in the inside of the housing; locating an end surface of the optical glass on a temperature measurement target material side adjacently to the surface of the temperature measurement target material and measuring the surface temperature of the temperature measurement target material; and providing a keeping member configured to keep a gap between the surface of the temperature measurement target material and the end surface of the optical glass on the temperature measurement target material side substantially constant; wherein the light receiving unit of the radiation thermometer receives the thermal radiation light emitted through the water that is present in the space between the surface of the temperature measurement target material and the end surface of the optical glass on the temperature measurement target material side, and wherein the surface of the temperature measurement target material on which the radiation thermometer detects the thermal radiation light is a plane that is substantially vertical to a horizontal direction and wherein the gap between the surface of the temperature measurement target material and the end surface of the optical glass on the temperature measurement target material side is 1.0 mm or shorter.