A vapor phase epitaxy apparatus having a processing chamber with a target workpiece such as a wafer being received therein is disclosed. This apparatus includes a first flow path which supplies the chamber with a gas needed for vapor phase epitaxial film formation on the wafer, and a valve unit for controlling the gas flow in the first flow path. The apparatus also includes a second flow path for supplying into the chamber a cleaner gas for cleaning of the interior of the chamber, and a third flow path for exhaust of residual gases within the chamber. The valve unit has a series combination of two-stage valves with a pressure switch being provided therebetween. An irregular gas mixture avoiding method for use with the apparatus is also disclosed.