For the achievement of data transfer time reduction, removal of noise data, and analytical efficiency improvement in an ADC data processing function of a time-of-flight mass spectrometer, the mass spectrometer comprises a data acquisition circuit including: an A/D converter; a signal intensity addition memory that stores data of ion signals such as a time range and the number of measurements and performs an addition process; a voltage value frequency addition memory that performs an addition process of frequencies of voltage values of the predetermined time range and the number of measurements and stores addition results; a threshold level computation circuit that computes a predetermined threshold level from the results in the memory; a compression memory that extracts only data exceeding the threshold level from the data in the signal intensity addition memory; and a counter that controls a measurement time for data acquisition and the operation of each circuit.