To provide a thin film deposition system and a thin film deposition method which enables a uniform supply of material gas by rapidly purging residual gas in a gas dispersion chamber and attaining high productivity.

A gas supply device so configured as to be coupled to a reaction chamber for gas exhausting includes a gas dispersion head for introducing gas into the chamber through the head surface. The gas dispersion head includes a first section for exhausting gas toward the susceptor direction through the head surface and a second section for exhausting gas toward the susceptor direction through the head surface. The first and second sections are separated from each other in the gas dispersion head and at least one of the sections is coupled to a gas exhaust system for purging gas present in the corresponding second section without passing through the head surface.