Abstract: A luminescence quantum efficiency measuring instrument is provided for easily and surely changing luminescence of a luminescent sample exhibiting strong luminescence anisotropy into an isotropic luminescence and for accurately measuring the luminescence quantum efficiency of the luminescent sample. The luminescence quantum efficiency measuring instrument comprises an integrating sphere (1) having a center, an excitation light entrance window (2), and a detection probe end (3) connected to a spectroscope, the excitation light entrance window and the detection probe end being disposed in respective directions perpendicular to each other on a plane including the center, wherein a luminescent sample (5) is disposed inside the integrating sphere (1) and on a vertical line extending from the center and vertical to the plane, and a baffle plate (7) is disposed at a place through which the luminescent sample (5) is seen from the detection probe end (3).