The invention relates to a prepreg, obtained by impregnating a base material with an epoxy resin composition containing an epoxy resin (A), a curing agent (B), an accelerator (C), a phenoxy resin (D), and an inorganic filler (E) and semi-hardening the impregnated material, wherein the inorganic filler (E) has an average particle diameter of 3 μm or less. When a circuit with a narrow wire distance is formed on a surface of a insulator substrate composed of such a prepreg by using a method of forming the circuit by plating process, an amount of the plating remaining on the insulator substrate surface at the circuit contour periphery can be reduced. As a result, it leads to stabilization of inter-circuit insulation resistance and increase in a yield during production of printed wiring boards.