There are obtained a processing method that allows adherence of foreign particles to an object to be processed in a load lock chamber to be suppressed, and a fabrication method of a semiconductor device using the processing method. The processing method includes the step of receiving a substrate that is the object to be processed at a load lock chamber (substrate load lock chamber) to load the substrate into a processing chamber where processing is to be applied to the substrate, and reducing internal pressure from a substrate load lock chamber 3. In the step of reducing internal pressure, pressure is released at a relatively low decompression rate, and then at a relatively high decompression rate.