PURPOSE: To calculate and set an optimum binarized threshold following up the variance of the illumination condition.

CONSTITUTION: Before the measurement processing in a visual recognition device, points P0 and P1 are obtained by an average density in a preliminarily determined monitor area and the optimum binarized threshold to determine a line L showing the relational formula between the density and the optimum binarized threshold with respect to the picture obtained in the illumination condition that illuminance is 0 and the picture obtained in the illuminance condition that illuminance capable of stable measurement processing is set. A control part measures the average density in the monitor area at intervals of a prescribed time, and, if the variance of the density is detected, the density is applied to the relational formula of the line L to calculate the optimum binarized threshold.