Disclosed is a calibrating apparatus which is adapted to remove a measurement deviation of a pyrometer, and more particularly, to an apparatus for calibrating a pyrometer, which calibrates a reference value so as to remove a deviation in a temperature measured in a pyrometer. The apparatus for calibrating a pyrometer includes a blackbody including a radiant space from which radiant energy is radiated, a body housing configured to receive the blackbody therein and including a light output wall having a light output port connected with the radiant space, a light output wall protecting cover comprising a transparent blocking plate disposed at a position opposite to the light output port so as to transmit a long wavelength of approximately 5 μm to approximately 20 μm may and configured to be coupled with the light output wall of the body housing, and a fixing member configured to fix the light output wall protecting cover to the light output wall of the body housing.