To provide a device that can measure flow potential easily with high accuracy from small diameter grain to large diameter grain with simple and compact constitution, without changing the flow of a fluid.

A fluid feeding container 11, a flow potential measuring cell 10 and a fluid receiving container 12 are communicated with one another under pressure-tight and airtight structure. The feeding container 11 is provided with a means for imparting constant high pressure, and the receiving container 12 is composed of a plurality of chambers 121, 122 that can be selectively partitioned in an airtight state by opening/closing mechanism 18. The volume of the receiving container 12 can be charged without changing a fluid passage by operation of the opening/closing mechanism 18.