A tandem quadrupole mass spectrometer comprising: a front-stage quadrupole mass filter for selecting, as precursor ions, ions having a specific mass-to-charge-ratio from among a variety of ions; a collision cell for causing the precursor ions to collide with a predetermined gas to dissociate the ions; a rear-stage quadrupole mass filter for selecting ions having a specific mass-to-charge-ratio from among a variety of product ions produced through the dissociation; and a detector for detecting the selected product ions, the tandem quadrupole mass spectrometer further comprising: a) a gas supplier for supplying the predetermined gas to an inside of the collision cell; characterized in that the tandem quadrupole mass spectrometer further comprises: b) a setting information memory for storing information on a gas pressure of the predetermined gas inside the collision cell or information for controlling a gas supply pressure of the predetermined gas, in association with a scan speed of a mass scan in a measurement mode in which the front-stage quadrupole mass filter performs the mass scan, wherein the stored information is set based on a relationship between peak separability and detection sensitivity, and the scan speed; and c) an analysis controller for controlling the gas supplier such that the gas pressure of the predetermined gas inside the collision cell accords with a scan speed of a mass scan to be performed, based on the information stored in the setting information memory, during execution of a measurement mode in which at least the front-stage quadrupole mass filter performs the mass scan, wherein the measurement mode in which the front-stage quadrupole mass filter performs the mass scan includes: a precursor ion scan measurement mode; a neutral loss scan measurement mode; and a measurement mode equivalent to a normal scan measurement mode in which only the front-stage quadrupole mass filter performs the mass scan while the rear-stage quadrupole mass filter does not perform ion selection according to a mass-to-charge-ratio. a) a gas supplier for supplying the predetermined gas to an inside of the collision cell; characterized in that the tandem quadrupole mass spectrometer further comprises: b) a setting information memory for storing information on a gas pressure of the predetermined gas inside the collision cell or information for controlling a gas supply pressure of the predetermined gas, in association with a scan speed of a mass scan in a measurement mode in which the front-stage quadrupole mass filter performs the mass scan, wherein the stored information is set based on a relationship between peak separability and detection sensitivity, and the scan speed; and c) an analysis controller for controlling the gas supplier such that the gas pressure of the predetermined gas inside the collision cell accords with a scan speed of a mass scan to be performed, based on the information stored in the setting information memory, during execution of a measurement mode in which at least the front-stage quadrupole mass filter performs the mass scan, wherein the measurement mode in which the front-stage quadrupole mass filter performs the mass scan includes: a precursor ion scan measurement mode; a neutral loss scan measurement mode; and a measurement mode equivalent to a normal scan measurement mode in which only the front-stage quadrupole mass filter performs the mass scan while the rear-stage quadrupole mass filter does not perform ion selection according to a mass-to-charge-ratio.