Tandem time-of-flight mass spectrometry method and apparatus permits an ion gate to be time set optimally at all times if the instrumental conditions are modified. Delayed extraction conditions for the mass-to-charge ratios of plural reference substances and optimum values of the time for which the ion gate is opened are measured and stored in a data table. Delayed extraction conditions and opening time of the ion gate which optimize the mass resolution at the mass-to-charge ratio of the desired precursor ions are found based on values stored in the table.