An arrangement for mechanically processing a workpiece having a position detecting pattern or patterns formed thereon has a mechanically processing tool disposed at a predetermined position, a worktable arranged so as to be linearly moved in at least one of the rectangular coordinate X- and Y-axes while holding the workpiece thereon, and a position aligning unit provided with an opto-electric line sensor capable of detecting one of the patterns of the workpiece and issuing pattern detected signals, and an operation/control unit capable of mathematically processing the detected signals from the line sensor to determine a position of the workpiece to be mechanically processed and generating control signals for controlling the movement of the worktable so that the position to be mechanically processed is aligned with the position of the mechanically processing tool. The line sensor may be a CCD type line sensor.