Provided is a projection-type charged particle optical system in which a projection magnification can be changed while a decrease in the accuracy in measuring a mass-to-charge ratio is being suppressed. A projection-type charged particle optical system according to the present invention includes a first electrode disposed so as to face a sample and having an opening formed therein for allowing a charged particle to pass, a second electrode disposed on a side of the first electrode opposite to where the sample is disposed and having an opening formed therein for allowing the charged particle to pass, and a flight-tube electrode disposed such that the charged particle that has been emitted from the sample and has passed through the second electrode enters the flight-tube electrode and being configured to form a substantially equipotential space thereinside. A principal plane is formed at at least two positions in a travel path of the charged particle.