An ion trap mass spectrometer is provided, including: an electron emitter; an ion trap storing ions generated by ionization resulting from an impact with electrons emitted from the electron emitter; a secondary ion filter for blocking out secondary ions generated due to ions selectively released by the ion trap; and a detector detecting ions selectively released from the ion trap, wherein the electron emitter, the ion trap, the secondary ion filter, and the ion detector are arranged on the same axis, so that a pure mass spectrum can be measured by excluding the secondary ions which are causes of background noise signals in the procedure of detection of the ions by the ion trap mass spectrometer.