A charged particle beam apparatus of the present invention comprises a transmission electron detector (113; 206) having a detection portion divided into multiple regions (201 to 205; 301 to 305), wherein a film thickness of a sample is calculated by detecting a transmission electron beam (112) generated from the sample when the sample is irradiated with an electron beam (109), as a signal of each of the regions in accordance with scattering angles of the transmission electron beam, and thereafter calculating the intensities of the individual signals. According to the above, there is provided a charged particle beam apparatus capable of performing accurate film thickness monitoring while suppressing an error due to an external condition and also capable of processing a thin film sample into a sample having an accurate film thickness, which makes it possible to improve the accuracy in structure observations, element analyses and the like.