PURPOSE: To improve total sensitivity and reduce differential sensitivity due to mass by properly applying optional voltage to an opening plate which restricts an ion beam on the ion exit side end of a quadrupole.

CONSTITUTION: Voltage is applied properly to an opening plate P for restricting an ion beam so as to form an electric field in the direction where an ion is accelerated between the ion beam restricting opening plate P located on the ion exit side end of a quadrupole QP and the electrode end of the QP. This opening plate P is connected to a variable voltage power supply V. The specified mass number ion proceeds in the arrow direction, passes through an open A, and is incident on an ion detector FC. When a positive ion is handled, the ion which passes between the QP electrodes is absorbed in the opening plate P by applying negative voltage to the opening plate P, accelerated, and passes through the QP end edge. As a result, sensitivity is improved and differential sensitivity can be reduced due to mass.