Impurity atoms with a magnetic moment are distributed rarely and randomly among the non-magnetic material in a spin glass. We suggest the Holtsmark model for description of spatial distribution of these atoms. Within the framework of the mean self-consistent field, the dependence of the spontaneous magnetic field on the temperature, as well as the critical temperature where this magnetic field disappears, have been found analytically. Keywords: Holtsmark distribution, critical temperature, spontaneous magnetic field.