An ideal delta function or a Gaussian function has been assumed for the photoconductive switching response of a photoconductive sampling experiment. In this letter, an effective wave form reconstruction method, called minimum phase retrieval algorithm, is presented to find the correct response function of the photoconductive sampling gate. In addition, the method is applied to the retrieval of a picosecond guided pulse wave form obtained from an on-wafer photoconductive sampling experiment. The reconstruction method should be used for the terahertz wave form detection measurement, where the pulse width is comparable to the switching time of the sampling gate. (C) 1998 American Institute of Physics.