A mass spectrometer and a mass spectrometry method having a high ion selection efficiency are provided. The mass spectrometer comprises an ion trap, a sample introducing device, an electron gun, a detector, a power supply for applying voltage to the ion trap, a control device for controlling the power supply and the electron gun, a mass analyzing device for performing mass spectrometry based on a detected signal of the detector. Using an auxiliary power supply, direction of an auxiliary electric field generated between end cap electrodes is made to point only toward the detector. In this occasion, by setting the cycle of the auxiliary voltage near the oscillation cycle of interest ion species in the axial direction, the interest ion species are synchronized with the auxiliary electric field to be certainly unstabilized in the detector side.