MS/MS mass spectrometer

The gas conductance on the ion injection side of a collision cell is made larger than the gas conductance on the ion exit side by providing two ion injection apertures in the collision cell. Due to the different gas conductances, a CID gas supplied through the gas supply tube generally flows in a direction from the ion injection side to the ion exit side in the collision cell, namely, in the ion's passage direction. When the ions injected in the collision cell slow down upon contacting with the CID gas, their progress is assisted by the gas flow, so that the delay of the ions in the collision cell is alleviated. As a result, it is possible to avoid a deterioration in the detection sensitivity of a target product ion and to prevent a ghost peak caused by the stay of the ions.