An apparatus for hydrophobic treatment of a semiconductor wafer comprises a tank in which HMDS liquid is stored, a process chamber in which the wafer is treated, and a unit for supplying HMDS liquid from the tank into the process chamber in an amount needed at any desired time. The process chamber can be decompressed by an ejector which is connected to the process chamber through an exhaust pipe. A mount on which the wafer is mounted is arranged in the process chamber and it includes a heater embedded therein. A ring surrounds the mount and two liquid receiving recesses are formed on the top of the ring. Two HMDS liquid supply pipes extend just above their corresponding liquid receiving recesses. HMDS in liquid phase is supplied into the process chamber and vaporized in it. The density of HMDS gas in the process chamber is controlled by adjusting the amount of HMDS liquid supplied.