into ionic currents at a collector 8 the ionic currents are output to a recorder 12 and a computer 14 through an amplifier 13 in this embodiment a sensitivity about three times higher than that of the prior art can be obtained fig3 shows a moving mechanism provided in the pressure gradient electrode 16 of fig2 though the cluster dissociation mechanism is the same as shown in the embodiment of fig2 in this embodiment the following effects can be obtained that is a bellows 17 is provided on the pressure gradient electrode 16 and an expanding contracting mechanism 18 of bellows 17 can be operated from the outside of the vacuum vessel so that an optimum pressure gradient can be set while actually measuring the ions fig4 and fig5 show spectra in the case that no cluster dissociation is carried out when trace amounts of ammonium and water are contained in a nitrogen gas and in the case that the cluster dissociation is carried out respectively that is fig4 shows the case of no cluster dissociation and fig5 the case of dissociation according to the present invention as is obvious from fig4 a peak of ammonirm nh 4 properly as a single peak is distributed into a plurality of peaks owing to the cluster formation reducing the s n ratio whereas in the present invention as is obvious from fig5 a substantially single proper ammonium peak can be obtained owing to the cluster dissociation improving the s n ratio according to the present invention the cluster ions inhibiting the higher sensitivity of an atmospheric pressure ionization mass spectrometer can be removed ty dissociation without loss in the amount of ions increase in the amount of a gas to.