A focal plane detection unit in an aligning system of a mask aligner is provided. Through sequential arrangement of a field stop, a focusing lens and a detector, an incident light beam penetrating the field stop is refracted to the detector through the focusing lens, and coordinates of the incident light beam projected on the detector are displayed through the detector and are calculated, and therefore the focal plane detection unit can timely, simply and conveniently calculate the defocusing amount of the incident light beam. The focal plane detection unit is disposed beside a beam-splitting element in an automatically-focusing aligning system, and therefore after light reflected or refracted from a mask or a workpiece enters the focal plane detection unit, the defocusing amount is calculated by the detector and outputted to a control unit, movement of a workbench is controlled by the control unit until the defocusing amount calculated by the focal plane detection unit is 0, and a clear alignment mark image is shown on an imaging sensor, thus finishing automatic focusing. Accordingly, the automatically-focusing aligning system is characterized by being timely, accurate and simple to operate.