We estimated J_c-B characteristics based on the sand-pile model from the magnetic field distribution on large disk samples with dimensions of 10×10×1 mm³. Such thin disks show a large demagnetizing effect. Thus simple estimates of J_c from the field distribution failed to determine correct values, and a large discrepancy was observed with J_c values determined from magnetization loops of small specimens. J_c-B characteristics obtained from the sand-pile model were in good agreement with those obtained from small specimens.