A layer forming method is disclosed which relies on reactive gas in a plasma state. The method includes steps of supplying power of not less than 1 W/cm2 at a high frequency voltage exceeding 100 kHz across a gap between a first electrode and a second electrode opposed to each other at atmospheric pressure or at approximately atmospheric pressure to induce a discharge, generating a reactive gas in a plasma state by the charge, and exposing a substrate to the reactive gas in a plasma state to form a layer on the substrate.