1. A mass spectrometer comprising: an ion source; a loop orbit; an ion detector; a flight time measurer for measuring a flight time of an ion flying the loop; a correction memory for storing correction information based on deviation of actual lengths of flight time of an ion having a known mass to charge ratio from calculated lengths of flight time at plural number of turns; and a mass calculator for determining a mass to charge ratio of an ion having an unknown mass to charge ratio using information of flight time of the unknown ion measured at plural number of turns, calculated lengths of flight time for the plural number of turns, and the correction information.