A mass spectrometer includes a collision cell (16) converging electrode (18), accelerating electrode (19) and front-side ion lens system (20) which is an electrostatic lens, which are all located within a medium-vacuum region, and a partition wall (22) for separating the medium-vacuum region from a high-vacuum region and an ion transport optical system (23) located within the high-vacuum region. Ions which have been extracted and accelerated by an accelerating electric field created between an exit electrode (16a) and the accelerating electrode (19) are focused into a micro-sized ion-passage opening (19a) by the converging electrode (18). The accelerating electrode (19) blocks a stream of gas, thereby decreasing the chance of contact of ions with gas particles behind the electrode. Additionally, the accelerating electric field imparts a considerable amount of kinetic energy to the ions, thereby preventing the ions from being dispersed even when they come in contact with the gas particles.