Time-of-flight mass spectrometer

A reflectron time-of-flight mass spectrometer including:

• a time-of-flight mass separator including a free flight space with no electric field and an ion reflector for reflecting an ion;
• an ion accelerator for accelerating an ion into the free flight space;
• and a detector for detecting an ion returning from a round trip in which the ion initially flies through the free flight space and once more flies through the free flight space after being reflected by the ion reflector, wherein:
• a shielding plate is placed in the free flight space, the shielding plate having a forward-side opening located on a forward-side ion path directed from the ion accelerator to the ion reflector and a return-side opening located on a return-side ion path directed from the ion reflector and the detector;
• and
• a width of the forward-side opening of the shielding plate in an extending direction of a plane on which both a central axis of the forward-side ion path and a central axis of the return-side ion path lie is smaller than a width of the return-side opening in the same direction.