PURPOSE: To estimate the total stock of a silo when a cavity occurs by comparing the total stock calculated from a flow rate meter with that calculated from an ultrasonic level meter to detect a cavity in the grains and applying the history control to various information on occurrence of the cavity to learn the correlation among these information.

CONSTITUTION: A silo stock control computer 5 receives the amounts of grains supplied into and discharged from a silo from the flow rate meters 1 and 2 and also receives the total stock of the silo from an ultrasonic level meter 3. Then the total stock calculated from the difference between the total amount of grains supplied and that discharged which are received from both meters 1 and 2 is compared with the total stock calculated based on the meter 3. When the difference between both total stocks is larger than a prescribed level, the coccurrence of a cavity is decided and the alarm information is displayed on a CRT screen. Meanwhile the computer 5 receives the brand of grains, the temperature, the water content, etc., from a field equipment controller 4 when the cavity occurs. Then the computer 5 applies the history control to various information when the cavity occurs and learns the correlation among these information to estimate the total stock of the silo and to display the total stock on the CRT screen when the cavity occurs.