A silicon nitride film having a thickness of 3 nm or less is formed on the surfaces of a P-well and N-well, as well as on the upper and side surfaces of a gate electrode, in which the silicon nitride film can be formed, for example, by exposing the surface of the P-well and N-well, and the upper and side surfaces of the gate electrode to a nitrogen-gas-containing plasma using a magnetron RIE apparatus. Then, pocket layers, extension layers and source/drain layers are formed while leaving the silicon nitride film unremoved.