A method of processing semiconductor substrates includes: depositing a film on a substrate in a reaction chamber; evacuating the reaction chamber without purging the reaction chamber; opening a gate valve and replacing the substrate with a next substrate via the transfer chamber wherein the pressure of the transfer chamber is controlled to be higher than that of the reaction chamber before and while the gate valve is opened; repeating the above steps and removing the substrate from the reaction chamber; and purging and evacuating the reaction chamber, and cleaning the reaction chamber with a cleaning gas.