A mass spectrometry method using a mass spectrometer, the mass spectrometry method comprising: calculating, by a control unit of the mass spectrometer, a theoretical mass spectrum by calculating, on the basis of a molecular formula of a component to be analyzed and an isotope abundance ratio of an element that is included in the component to be analyzed and for which a plurality of isotopes exist, a mass of the isotopes of the component to be analyzed and an abundance ratio of the component to be analyzed for each mass; and ionizing, by a preprocessing unit of the mass spectrometer, a measurement target, detecting, by a mass detection unit of the mass spectrometer, a mass of ionized ions and the number of ions in each mass; calculating, by the control unit, a first mass spectrum on the basis of a detection result of the mass detection unit; operating, by the control unit, the preprocessing unit and the mass detection unit in a state where the measurement target is absent; calculating, by the control unit, a second mass spectrum when the measurement target is absent on the basis of the detection result of the mass detection unit; and subtracting, by the control unit, the second mass spectrum from the first mass spectrum so as to obtain a subtracted mass spectrum; calculating, by the control unit, a degree of matching between the subtracted mass spectrum and the theoretical mass spectrum for only a mass in which a peak of the theoretical mass spectrum exists; determining, by the control unit and on the basis of the degree of matching, presence or absence of the component to be analyzed in the measurement target; wherein in the determination, the control unit compares the degree of matching with a first threshold and a second threshold larger than the first threshold, determines that the component to be analyzed is absent when the degree of matching is smaller than the first threshold, and determines that the component to be analyzed is present when the degree of matching is equal to or larger than the second threshold. calculating, by a control unit of the mass spectrometer, a theoretical mass spectrum by calculating, on the basis of a molecular formula of a component to be analyzed and an isotope abundance ratio of an element that is included in the component to be analyzed and for which a plurality of isotopes exist, a mass of the isotopes of the component to be analyzed and an abundance ratio of the component to be analyzed for each mass; and ionizing, by a preprocessing unit of the mass spectrometer, a measurement target, detecting, by a mass detection unit of the mass spectrometer, a mass of ionized ions and the number of ions in each mass; calculating, by the control unit, a first mass spectrum on the basis of a detection result of the mass detection unit; operating, by the control unit, the preprocessing unit and the mass detection unit in a state where the measurement target is absent; calculating, by the control unit, a second mass spectrum when the measurement target is absent on the basis of the detection result of the mass detection unit; and subtracting, by the control unit, the second mass spectrum from the first mass spectrum so as to obtain a subtracted mass spectrum; calculating, by the control unit, a degree of matching between the subtracted mass spectrum and the theoretical mass spectrum for only a mass in which a peak of the theoretical mass spectrum exists; determining, by the control unit and on the basis of the degree of matching, presence or absence of the component to be analyzed in the measurement target; wherein in the determination, the control unit compares the degree of matching with a first threshold and a second threshold larger than the first threshold, determines that the component to be analyzed is absent when the degree of matching is smaller than the first threshold, and determines that the component to be analyzed is present when the degree of matching is equal to or larger than the second threshold.