An apparatus for growing a compound semiconductor layer by metal organic chemical vapor deposition (MOCVD) is disclosed. Utilizing the apparatus, the semiconductor layer of uniform thickness and uniform composition can be grown. The apparatus includes a plurality of vent pipes which spout a mixed gas of source material gases and a dilution gas into a reactor chamber, vertical to a substrate surface. The apparatus also includes a gas supply system in which a gas flow rate through each vent pipe is made to be controllable individually by a flow controlling device. In addition, a controller is operatively connected to the flow controlling devices, so that automatic growth of a semiconductor layer of a high quality can be achieved. When two source material gases used are mutually too reactive and deposits are formed within the gas supply system, two separate gas supply systems for these two gases are demonstrated to be effective. Another embodiment which enables individual control of concentration of the source material gas and the gas flow rate through each vent pipe, is also disclosed.