The present triple quadrupole mass spectrometer determines the relationship between a parameter, such as the mass-to-charge ratio of a precursor ion or that of a product ion, and the optimal collision-gas pressure giving the highest signal intensity in an MRM measurement, derives an approximate equation expressing that relationship, and stores the information representing the equation in an optimum collision-gas pressure calculation information storage section. When a measurement is to be performed, an analysis operator enters the mass-to-charge ratio of a precursor ion or product ion of a target compound. Based on the approximate equation read from the storage section, an optimum collision-gas pressure calculator determines the optimum collision-gas pressure for the specified precursor ion or product ion, and sets this pressure as a measurement condition for the apparatus.