A mark detecting method of detecting a notch as a mark formed on the outer circumference of a wafer held on a holding table. The mark detecting method includes the steps of index-rotating the holding table to image at least three points on the outer circumference of the wafer and to thereby detect the coordinates at the three points on the outer circumference of the wafer, calculating the center of the wafer from the coordinates at the three points, centering the wafer with respect to the holding table, and continuously rotating the holding table through 360° to image the whole of the outer circumference of the wafer by using a minimum imaging area corresponding to the outer circumference of the wafer and to thereby detect the angle where the notch is located.