A cloud of ions captured in an ion trap (2) is irradiated with a stream of hydrogen radicals cast through a radical particle introduction hole (26) bored in a ring electrode (21) at a flow rate of 4×1010 [atoms/s]. As a result, a radical induced dissociation which does not rely on the transfer or capture of electrons occurs within the ion trap (2), whereby c/z-type fragment ions are efficiently generated. After the irradiation with the hydrogen radicals, a supplemental collision-induced dissociation process may performed by introducing inert gas into the ion trap (2) and resonantly exciting the ions, in order to further promote the generation of the c/z-type fragment ions. In this manner, according to the present invention, it is possible to achieve radical induced dissociation of singly-charged ions derived from a peptide and use the thereby generated c/z-type fragment ions for mass spectrometry.