PURPOSE: To enable an ion detector composed of a combination of a CD and electronic booster to detect negative ion with a good efficiency.

CONSTITUTION: A CD 4 is arranged onto a passage of ion I to be detected, and to the CD 4 a high voltage power source 5a and a power source 5c are connected switchably through a switch S. An electronic booster 1 of which incident face has earth potential is provided in the normal line of the CD 4. In detecting positive ion I negative high voltage of about 5KV is applied from the high voltage power supply 5a to the CD 4 to make ion impinge on the CD 4 to emit secondary electron E. Then, the secondary electron is accelerated by the voltage between the CD 4 and an electron booster and applied onto the electronic booster. In detecting negative ion I relatively low negative voltage of about 500V is impressed to the CD4 form the power source 5c to make negative ion impinge on the CD 4. In the case of minus ion detection, by the application of low electric potential to the CD 4, secondary electron is discharged from the CD 4 and applied to the electronic booster 1 as the similar way in the case of positive ion detection.