The variation of the relative permittivity of charged dielectrics with trapped charge density has been investigated by a time-resolved current method, in conjunction with a mirror image method employing a scanning electron microscope. The calculation is made by a mathematical expression derived from classical electromagnetic theory. It is found that the relative permittivity of the charged area in the polymethylmethacrylate sample increases with the trapped charge density and saturates at a certain value of the trapped charge density. These observations have been discussed by analogy with the dielectric saturation occurring at a high applied external electric field.