PURPOSE: In a sweep type mass spectrometer, to improve S/N and the sensibility by arranging micro detector array having the space resolution at the converging point of the ion beam thereby accumulating the detection signal synchronously with the mass number sweeping.

CONSTITUTION: The specimen ion 2 generated from an ion source 1 is dispersed in accordance to the ration between the mass and the charge through a double focus analyzing system comprising an electric field E and a magnetic field M to be sweeped. In a channel plate 5 arranged at the center of said double focus point, the incident ion is multiplied and provided to said detector array 6. Said array 6 has many micro collectors and the incident charge amount P is read out by a read-out control circuit 7 then A/D converted 8 and provided to a computer 9. The electrode number (n) under reading-out is provided from said circuit 7 to a converter 12, then the mass number MO from the converter 13 is multiplied 14 and provided to the computor 9. The computer 9 will determine the mass number to distribute the signal P to the address of the memory 15 set in accordance to the mass number which is added to the existing value.