To simplify the construction on the hopper side of a measuring instrument, brackets (24 and 34) are fitted to gates (22 and 32) of a pool hopper (8) or a measuring hopper (9), and rollers (28 and 38) are disposed at their extension portions (24a and 34a). Operation arms (44 and 64) having bifurcated distal end portions are provided to a measuring instrument main body B, and the rollers (28 and 38) of the hoppers (8 and 9) fitted to the main body side B are engaged in such a manner as to be gripped by the operation arms (44 and 64). When the operation arms (44 and 64) are swung around support shafts (43 and 63), the gates (22 and 32) of the hoppers (8 and 9) are swung around the support shafts (25 and 35) through the rollers (28 and 38), and lower discharge ports (21b and 31b) are opened/closed.