A time of flight type mass spectrometer (TOF-MS) of the present invention includes: a flight space containing a loop orbit on which ions fly once or more than once; a flight controller for making ions of a same mass to charge ratio fly the loop orbit at several values of number of turns; a flight time measurer for measuring a length of flight time of the ions; and a processor for determining the mass to charge ratio of the ions based on a relationship between the value of number of turns and the length of flight time of the ions. The speed of ions flying a loop orbit depends on their mass to charge ratios. For ions of the same mass to charge ratio, the difference between the lengths of flight time of the ions flying the loop orbit N turns and of the ions flying the loop orbit N+1 turns depends on the speed of the ions, so that the difference depends on the mass to charge ratio of the ions. The difference in the length of flight time is unrelated to the variation in the starting time (jitter), variation in the detection timing (jitter), etc, so that the value of the mass to charge ratio can be precisely determined free from errors caused by such disturbing factors.