A plasma doping method capable of introducing impurities into an object to be processed uniformly is supplied. Plasma of a diborane gas containing boron, which is a p-type impurity, and an argon gas, which is a rare gas, is generated, and no bias potential is applied to a silicon substrate. Thereby, the boron radicals in the plasma are deposited on the surface of the silicon substrate. After that, the supply of the diborane gas is stopped, and bias potential is applied to the silicon substrate. Thereby, the argon ions in the plasma are radiated onto the surface of the silicon substrate. The radiated argon ions collide with the boron radicals, and thereby boron radicals are introduced into the silicon substrate. The introduced boron radicals are activated by thermal processing, and thereby a p-type impurity diffusion layer is formed in the silicon substrate.