An electron microscope comprising: an electron beam source for producing an electron beam; an electron beam detector for detecting a part of the electron beam; a DC (direct current) component extractor for extracting a DC component of an effective value of a detection signal emanating from the electron beam detector; an image detector for detecting a signal produced in response to impingement of the electron beam on a sample; and an arithmetic section for performing division on an image signal emanating from the image detector and a detection signal emanating from the electron beam detector and multiplication between a quotient signal indicative of the result of the division and the signal extracted by the DC component extractor, wherein said DC component extractor comprises: an effective value computing section for computing said effective value; an AC component extractor for extracting an AC component of a signal indicative of the computed effective value; and a subtractor for performing a subtraction between the signal indicative of the computed effective value and the signal extracted by said AC component extractor, characterised in that the electron microscope further comprising a processing section; wherein said AC component extractor has a capacitor to which a signal indicative of the computed effective value is applied, a first resistor, a second resistor having a smaller resistance value than the first resistor, and switching means for switching a resistor, which is connected with the capacitor and forms a high pass filter, between the first resistor and the second resistor; and wherein said processing section performs an operation to control the switching means such that the resistor connected with the capacitor is switched from the first resistor to the second resistor when a given operation of the electron microscope is detected and an operation to control the switching means such that the resistor connected with the capacitor is switched back to the first resistor from the second resistor at the instant when a given time has passed since the switching from the first resistor to the second resistor. an electron beam source for producing an electron beam; an electron beam detector for detecting a part of the electron beam; a DC (direct current) component extractor for extracting a DC component of an effective value of a detection signal emanating from the electron beam detector; an image detector for detecting a signal produced in response to impingement of the electron beam on a sample; and an arithmetic section for performing division on an image signal emanating from the image detector and a detection signal emanating from the electron beam detector and multiplication between a quotient signal indicative of the result of the division and the signal extracted by the DC component extractor, wherein said DC component extractor comprises: an effective value computing section for computing said effective value; an AC component extractor for extracting an AC component of a signal indicative of the computed effective value; and a subtractor for performing a subtraction between the signal indicative of the computed effective value and the signal extracted by said AC component extractor, an effective value computing section for computing said effective value; an AC component extractor for extracting an AC component of a signal indicative of the computed effective value; and a subtractor for performing a subtraction between the signal indicative of the computed effective value and the signal extracted by said AC component extractor, characterised in that the electron microscope further comprising a processing section; wherein said AC component extractor has a capacitor to which a signal indicative of the computed effective value is applied, a first resistor, a second resistor having a smaller resistance value than the first resistor, and switching means for switching a resistor, which is connected with the capacitor and forms a high pass filter, between the first resistor and the second resistor; and wherein said processing section performs an operation to control the switching means such that the resistor connected with the capacitor is switched from the first resistor to the second resistor when a given operation of the electron microscope is detected and an operation to control the switching means such that the resistor connected with the capacitor is switched back to the first resistor from the second resistor at the instant when a given time has passed since the switching from the first resistor to the second resistor.