A charged particle beam instrument is offered which has a specimen pre-evacuation chamber. An outflow flow rate adjusting valve (70) adjusts the flow rate of gas exhausted from the specimen pre-evacuation chamber (20) under control of controller (82). The controller (82) for making a decision as to whether the difference between a first pressure in the pre-evacuation chamber (20) before the adjusting valve (70) is controlled to the first degree of opening and a second pressure in the pre-evacuation chamber (20) after the adjusting valve (70) has been controlled to the first degree of opening is greater than a first reference value.