The origin of an exponential distribution of localized states in metallophthalocyanines (MPcs) has been studied on cobalt phthalocyanine (CoPc) and iron phthalocyanine (FePc) dispersed in polymer binders in terms of transient photoconductivity measurements. It is found that the localized-state distributions, calculated from transient photoconductivity data, are characterized by an exponential distribution on which a Gaussian distribution is superimposed. It is also found that the characteristic temperature of the exponential distribution in CoPc and FePc is dependent on their dielectric constants. It is shown that this experimental finding is in agreement with the theoretical prediction deduced from electrostatic potential fluctuations caused by MPc+ and O-2(-) in the MPcs. From the similarity found in the localized-state distributions of other MPcs, it is suggested that such electrostatic potential fluctuation is a common origin of an exponential distribution of localized states in MPcs.