A gap detection apparatus for determining in real time the gap required for electrochemical machining gap includes a tooling electrode, a plurality of tool adjusting electrodes, a feedback circuit, a processing feed mechanism for controlling the tooling electrode, a three-dimensional driving mechanism, and an automatic control and measurement system. The tooling electrode includes a plurality of through-holes for receiving tool adjusting electrodes. The three-dimensional driving mechanism is mounted upon the processing feed mechanism, which includes a Z-coordinate feeding portion having a thimble for the feeding of the tool adjusting electrodes. The automatic control and measurement system controls the feed of the processing feed mechanism and the three-dimensional driving mechanism, and establishes the required gap for electrochemical machining.