PURPOSE: To make both of positive and negative ions detectable without any switching operation, by locating a deflecting electrode combined with ion-ion converter in front of an electron multiplier tube, while impressing voltage in constant polarity to this converter so as to make it act as a deflecting electrode for an ion in the same polarity and act as a combined converter for another ion in the opposite polarity.

CONSTITUTION: A secondary electron multiplier tube P consists fo a first dynode 2 and the plural number of stages of dynodes 3 and the subsequent ... of the secondary electron multiplier tubes and an anode 4, and a deflecting electrode combined with ion-ion converter 1 is located in front of this secondary electron multiplier tube P. A power source 5 for impressing voltage on a dynode is so designed that a voltage of for example -0.7W3.5KV can be impressed on the first stage dynode, and, a gain can be changed as this voltage changes. In addition, an impressed voltage power source 6 for the converter 1 is installed and thereby a constant polarity voltage of 0W+3.5KV, for example, is impressed on the converter 1 in design.