A method and apparatus in which separate steps are used between feeding source gases and growing films in a process of chemical vapor deposition (CVD) for compound semiconductor films. In one embodiment, a CVD reactor chamber has a piston which can change the volume of the chamber to control the pressure of the source gases therein. After source gases are fed to the chamber having a substrate under the condition that no CVD takes place due to an insufficient vapor pressure, the chamber is kept closed for a few seconds, and then pressurized by the piston to start CVD. A typical result indicates that a Hg.sub.1-x Cd.sub.x Te (where x=0.2) film on a 3-inch CdTe wafer has only 1% (or .DELTA.x=0.002) inhomogeneity in composition.