PURPOSE: To make all grains confluent always and uniformly and to enable conveyance by setting time difference successively at the timing for start of discharge from each preliminary tank for mixing.

CONSTITUTION: Plural preliminary tanks T1WT5 for mixing are provided in parallel with the upper face of a conveyor 3 and discharge valves B1WB5 which can vary the discharge amount are provided to the discharge ports of each tank and also a volumetric weight measuring device 1 which measures the volumetric weight of each mixed grain is provided. An arithmetic control device 5 calculates each number of revolutions of valves B1WB5 with both the volumetric weight of each mixed grain measured by the device 1 and the inputted mixing ratio of grains and thereby each grain is discharged from tanks T1WT5 at the set mixing ratio by weight. Furthermore, valves B1WB5 are connected to the arithmetic control device 5 so that respective valves start to actuate successively with the time differential for the conveyance direction of the conveyor 3. As a result, all grains are made confluent always and uniformly and conveyed and the discharge amount from each preliminary tank for mixing is controlled and the prescribed uniform mixing can be performed consistently.