PURPOSE: To enable automatic detection of the change in fluidity and of the time required by the change by discriminating whether or not a light-shading plate shades the light from a light projector to a light-receiving element.

CONSTITUTION: A sampling tube 1 is fixed to a supporting stand 2 rotatable around an axis 3 and is held for a time t1 set for start of tipping in such a condition that one end 1b of the tube 1 is positioned lower than the other end 1a and that the light from a light projector 7 to a light-receiving element 8 is not shaded by a plate 2a. Next, the stand 2 being rotated by one end 5b of a lever 5, the tube is held for a time t2 for continuation of tipping in such a condition that the one end 1b is positioned higher than the other end 1a and that the light from the projector 7 to the light-receiving element 8 is shaded by the plate 2a. And, after the lapse of the time t2, the lever 5 is removed from the stand 2, and the change in fluidity of a sample is discriminated according to whether the plate 2a shades the light from the projector 7 to the light-receiving element 8 or not.