To provide an identifying method of a candidate parent ion suitable to identify the candidate parent ion in a real time and to use for a chromatography.

An improving method of a parent ion scanning is disclosed. In one embodiment, a quadruple mass filter 3 of an upstream of a collision cell 4 is arranged to work in a high pass mode. The parent ion passed the mass light filter 3 is fragmentized by a collision cell 4, and is discovered by an orthogonal type time of flight analyzer 5 to acquire daughter ion spectra. Ions having mass/charge ratio of not more than cutoff value of the mass light filter 3 are identified as the daughter ions, and the candidate parent ion is subsequently discovered, and by acquiring corresponding daughter ion spectra, their identifications can be confirmed. In the first embodiment, because the collision cell is switched in alternation in a space of a high or low fragmentation, and further, the candidate parent ion can be identified based on a dissipation of a prescribed ion or a neutral particle.