PURPOSE: To improve selective operation efficiencies, by automatically adjusting the clearance between hulling rolls based on the result of compatative operation between a measured hulling rate and set hulling rate, thereby automatically adjusting the clearance to a roll clearance corresponding to the efficiency when the operation efficiency is changed.

CONSTITUTION: A part of mixed rice introduced as sample grains is detected by means of a unhulled rice-rice judging sensor 19, whereby a CPU 30 having received the judgement informations judges the difference between unhulled rice and unmilled rice and stores them to compute a hulling rate, wherein measured hulling rates and hulling rate having been set beforehand are computed in comparison with each other, in response to the result of which actuation command signals are outputted to a positive rotation circuit 33 or to a reverse rotation circuit 34 so that the clearance between rolls is automatically controlled to achieve a desired hulling rate. During such controlled operation, the CPU 30 reads a change in the position of a milled rice adjustment valve to deliver an actuation command signal to a proper circuit, the positive circuit 33 or reverse rotation circuit 34, when it has judged the occurrence of change, thereby adjusting automatically the roll clearance in response to a variation in the operation.