When a time-of-flight mass selector having a chopper using a deflector selects the masses of the ions, an ion beam is deflected. As a result, at least a part of the ion beams diagonally pass through an aperture electrode with respect to the axis. Accordingly, there has been a problem that a position on an object irradiated with a cluster ion beam, results in moving. This mass selector includes: a flight tube having an equipotential space that makes a charged substance fly therein; a deflector that is installed in a downstream side with respect to the flight tube in a direction in which the charged substance flies; a first aperture electrode that is installed in a downstream side with respect to the deflector in a direction in which the charged substance flies; and a second aperture electrode that is installed in between the deflector and the first aperture electrode.