To provide a charged particle beam apparatus and a pressure measuring method for the charged particle beam apparatus, capable of measuring a vacuum pressure value around its charged particle gun.

Values R1-R8 corresponding to conductances of an accelerating tube 3, an exhaust tube 6 and an ion pump 7, and values I2-I8 corresponding to the amount of gas exhaustion of the accelerating tube 3 and the exhaust tube 6 are input through an input means 23 by an operator. A value of ion current i0 flowing in the ion pump 7 is input to a computing means 18 as a value Vs, and a vacuum pressure value P0 measured by a vacuum gage 9 is input to the computing means 18 as a value Vg. The amount of gas exhaustion (I1) near an electron gun 1 is obtained by an exhaustion gas amount calculating means 19 which calculates it using data (Vg, Vs, R7, R8 and I2-I8). Then, the vacuum pressure value (Vt) around the electron gun 1 is obtained by a pressure calculating means 20 which receives and calculates the data (I1) and the above-mentioned data (Vg, R1-R6 and I2-I6).