A mass spectrometer having a multi-stage differential pumping system with an ion lens provided in a partition wall separating a second intermediate vacuum chamber and a third intermediate vacuum chamber, the incircle radii of ion guides and the size of the opening of the ion lens are determined so that the circumferential edge of the opening is located outside the circumferential surface of a virtual tubular body straightly connecting the incircle at the rear edge of the second ion guide in the front stage and the incircle at the front edge of the third ion guide in the rear stage. Although the ion lens is located in between, the radio-frequency electric field created by the second ion guide can be effectively connected to the radio-frequency electric field created by the third ion guide through the opening of the ion lens.