An apparatus for measuring an overall heat transfer coefficient may include an external chamber provided using a heat insulation material and including an internal space, an internal chamber disposed in the external chamber and having an opening upper portion, a heat source supply to supply heat to an internal portion of the internal chamber, a cutoff portion disposed in the upper portion of the internal chamber to seal the internal chamber and prevent an outflow of heat emitted from the heat source supply, and a temperature measurement portion disposed in an internal portion and an external portion of the internal chamber to measure an internal temperature and an external temperature of the internal chamber, in which the cutoff portion may realize covering conditions indoors through a combination of a covering material and a thermal screen, and adjust the external temperature of the internal chamber.