To measure the specific gravity of slurry easily and correctly and dispense with maintenance by feeding slurry at always constant specific flow velocity to a specific gravity measuring container of round pipe shape, provided separately from a slurry tank, to detect weight.

A mixer 3 for agitating slurry 2 is fitted to a slurry tank 1 for storing the slurry 2 to be fed between a polished glass article and a polishing tool. The slurry 2 is fed from the tank 1 to a specific gravity measuring container 7 by a pump 4 and returned to the tank 1 via an outflow pipe 8 and an outflow hose 9. An inflow port (outflow port) is formed into gently enlarged (narrowed) cross-sectional area shape so as not to generate complex flow such as separating bubbles when the slurry 2 flows in or flows out of the container 7, and the relation between the flow velocity V1 in the container 7 and the average settling velocity V2 of abrasive grains in the container 7 is made V1≤9V2. The weight of the container 7 is detected by a load cell 11 in the state of the distance from the suspension center being as short as possible so as to reduce drift.