An alignment detection apparatus for detecting the alignment of a superfine exposure system including an interrupted emission light source. The apparatus is equipped with an output variation monitoring sensor to detect an output variation of the light source and further a photoelectric conversion device to perform the photoelectric conversion of the alignment light to generate discrete photoelectric conversion signals. Also included in the apparatus is an integration and hold circuit to integrate and peak-hold each of the photoelectric conversion signals. This peak-hold values are corrected on the basis of the detected light-source output variation due to the monitoring sensor to output stepwise continuous signals from which a harmonic component is removed by a filter so as to obtain a smooth analog waveform. This analog waveform is processed by a phase detector. The output of the phase detector allows an accurate alignment irrespective of using the interrupted emission light source.