PURPOSE: To measure a refining degree of grain accurately, by providing a standard white board in a sample room and also providing a transmitted light measuring device and a reflected light measuring device.

CONSTITUTION: The face of a side of an integral sphere 1 and its opposite face of a detection part chamber 2 are made of quartz glass 18 and the other one side face is provided with an opening part allowing a guide pole 20 attached with a cleaner freely going in and out. A standard white board 22 to be used as the standard for measurement of a refining degree is provided to the pole 20. The pole 20 is inserted into the chamber 2 until the board 22 reaches to the front face of a transmitted light photodetecting element 8. At first, a dark electric current of a reflected light photodetecting element 8' is measured in a state not introducing the light and next, the light is made incident into the board 22 by opening a light introducing shutter 7 and then, an amplification factor is adjusted so that a degree of whiteness of the board 22 is set as the standard degree of whiteness. Hereafter, the pole 20 is drawn out and the sample is introduced into the chamber 2 and then, the refining degree is found by measuring reflactance and transmittance.