Abstract: A time-of-flight ion mass analyzer comprises a device for preacceleration of ions being analyzed and a sensing member of the analyzer detecting the moment at which an ion enters a time-of-flight space, a reflector and an electron detector connected to a time interval measurement device, which are provided in series downstream the preacceleration device. The reflector has at least two grid electrodes: an intermediate electrode and a bottom electrode. The intermediate electrode is designed to receive a potential which creates, in the reflector space, two zones divided in accordance with electric field steepness, and a difference of potentials between the bottom electrode of the reflector and the sensing member of the analyzer is chosen in the absolute value such as to be greater than, or equal to, the potential difference at the preacceleration device.