A charged particle beam apparatus comprising: a sample holder for holding a sample; a particle source for directing a particle beam at the sample held to the sample holder a vacuum chamber (2, 2A) having an interior space where the sample holder is positioned; an evacuation section for venting gas in the interior space of the vacuum chamber; a vacuum gauge for measuring the pressure in the interior space of the vacuum chamber; a heater for heating the sample holder; a temperature sensor for detecting the temperature of the sample holder; a gas inlet assembly (14, 17, 19) for admitting a dry gas containing no moisture into the interior space of the vacuum chamber; and a control section which, in operation, is configured to: control supply of electric power to the heater to initiate heating of the sample holder using the heater; determine whether a temperature of the sample holder has reached or is above a preset temperature; during controlling supply of electric power and determining whether a temperature of the sample holder has reached or is above a preset temperature, drive the evacuation section to vent the gas in the interior space of the vacuum chamber; when the temperature is determined to have reached or is above the preset temperature, acquire information about the pressure in the interior space of the vacuum chamber (2, 2A) from the vacuum gauge; based on the pressure information acquired, determine whether the pressure in the interior space of the vacuum chamber (2, 2A) has reached below a given pressure, and when the pressure in the interior space has reached below a given pressure, control the gas inlet assembly (14, 17, 19) based on the acquired information about the pressure in the interior space measured by the vacuum gauge so as to admit the dry gas into the vacuum chamber. a sample holder for holding a sample; a particle source for directing a particle beam at the sample held to the sample holder a vacuum chamber (2, 2A) having an interior space where the sample holder is positioned; an evacuation section for venting gas in the interior space of the vacuum chamber; a vacuum gauge for measuring the pressure in the interior space of the vacuum chamber; a heater for heating the sample holder; a temperature sensor for detecting the temperature of the sample holder; a gas inlet assembly (14, 17, 19) for admitting a dry gas containing no moisture into the interior space of the vacuum chamber; and a control section which, in operation, is configured to: control supply of electric power to the heater to initiate heating of the sample holder using the heater; determine whether a temperature of the sample holder has reached or is above a preset temperature; during controlling supply of electric power and determining whether a temperature of the sample holder has reached or is above a preset temperature, drive the evacuation section to vent the gas in the interior space of the vacuum chamber; when the temperature is determined to have reached or is above the preset temperature, acquire information about the pressure in the interior space of the vacuum chamber (2, 2A) from the vacuum gauge; based on the pressure information acquired, determine whether the pressure in the interior space of the vacuum chamber (2, 2A) has reached below a given pressure, and when the pressure in the interior space has reached below a given pressure, control the gas inlet assembly (14, 17, 19) based on the acquired information about the pressure in the interior space measured by the vacuum gauge so as to admit the dry gas into the vacuum chamber. control supply of electric power to the heater to initiate heating of the sample holder using the heater; determine whether a temperature of the sample holder has reached or is above a preset temperature; during controlling supply of electric power and determining whether a temperature of the sample holder has reached or is above a preset temperature, drive the evacuation section to vent the gas in the interior space of the vacuum chamber; when the temperature is determined to have reached or is above the preset temperature, acquire information about the pressure in the interior space of the vacuum chamber (2, 2A) from the vacuum gauge; based on the pressure information acquired, determine whether the pressure in the interior space of the vacuum chamber (2, 2A) has reached below a given pressure, and when the pressure in the interior space has reached below a given pressure, control the gas inlet assembly (14, 17, 19) based on the acquired information about the pressure in the interior space measured by the vacuum gauge so as to admit the dry gas into the vacuum chamber.