Abstract: In an automatic door system and a method for controlling an automatic door, the automatic door is not opened when a passerby just passes by the door. A data processing device (5) produces a differential image by subtracting an initial image from a detected image on the basis of digital data entered from a subtractor (4), and judges whether a gray-scale shadow is in the differential image or not. When a gray-scale shadow appears in the differential image, the data processing device (5) calculates the maximum width (Xm) of the gray-scale shadow in the X direction to compare it with a predetermined threshold value (Xth). When the maximum width (Xm) is equal to the threshold value (Xth) or more, the data processing device (5) judges that a passerby faces toward an automatic door (1), and outputs a driving control signal (C) for opening the automatic door (1).