The present invention relates to a sterilization apparatus (1) for sterilization of objects, said sterilization apparatus (1) comprising: a chamber (2) for receiving said objects to be sterilized, said chamber (2) having a sterilization medium inlet (3a) and a sterilization medium outlet (3b)； an inlet flow control device (14) connected to said sterilization medium inlet (3a) for controlling a supply of a sterilization medium to said chamber (2)； a drain system (6) connected to said sterilization medium outlet (3b) of the chamber (2) for controllable discharge of fluid from said chamber (2)； and a control unit (12) connected to said inlet flow control device (14) and said drain system (6) and configured to control said inlet flow control device (14) and said drain system (6) in accordance with a predefined sterilization scheme； wherein the drain system (6) comprises: a drain inlet (60) connected to said sterilization medium outlet (3b) of the chamber (2)； a drain outlet (62) connected to a vacuum system (8) for evacuating the chamber (2)； a first conduit (64) connecting said drain inlet (60) and said drain outlet (62)； and a proportional valve (66) arranged along said first conduit (64) said proportional valve (66) being controllable by said control unit (12) to partly restrict fluid flow through said first conduit (64) according to a predefined continuous relation between a control parameter and a valve opening of said proportional valve (66).
