PURPOSE: To provide a method and device for measuring a change in setting time based on a change of quality of a gypsum in use without flawing the surface of the gypsum in the production of a gypsum building material or a cured gypsum body using a hemihydrate gypsum to be used for managing a production process of a gypsum building material to enhance a productivity and stabilize a quality.

CONSTITUTION: A surface of paste or slurry prepared by kneading a hemihydrate gypsum singly or together with glass fibers or the like is smoothed, and a change in unreacted water content existing on the surface of the slurry with time is measured by a change of reflectance of emitted light to determine a setting state. A device therefor comprises a light source part 1 consisting of a light source 2 and a condensing lens 3 and a measuring part consisting of a sensor 5 detecting a light emitted from the light source part 1 and reflected by the surface of the slurry and an indicator 6 indicating the output of the sensor 5.