PURPOSE: To control the adjustment of light quantity within a short time by executing initial adjustment for changing the light quantity of a light emitting element so that the output level of a light receiving element becomes constant at the initial time when no slided rice flows.

CONSTITUTION: While allowing slided rice S from an unhulled rice sliding device to flow into a sensor light projection area in which light is projected from the light emitting element to the light receiving element 2, a hull removing rate sensor 5 finds out a threshold K which is a boundary between unhulled rice 'momi' M and unpolished rice 'genmai' G and calculates a hull removing rate by a transmitted light quantity distribution 4 of the sensor projecting light. A hull removing rate control device 11 executes initial adjustment for changing the light quantity of the element 1 so that the output level of the element 2 becomes constant at the initial time when no slided rice S flows and sets up the obtained quantity of light emission as the starting light quantity for light quantity adjustment to be executed after the flow of the slided rice S. Since the quantity of light emission due to the element 1 is included in the proper light receiving quantity range L of the element 2 by the initial adjust ment from the start of the light quantity adjustment control when the slided rice starts to flow, the light quantity adjustment control can be accurately and rapidly executed.