PURPOSE: To make it possible to availably adapt the titled analytical apparatus even to an unvolatile or a thermally unstable specimen, by irradiating high energy fission fragment emitted from 252 CF.

CONSTITUTION: Among fission fragements emitted in every direction from 252 CF as a center, two fission fragments are utilized and one direction is detected by a detecting electrode 13 while a specimen is ionized by the fission fragment to an 180° direction with respect to said direction while this ion is detected by other detecting electrode 14 to make it possible to know the mass of the ion by the difference of detecting times. At this time, the signal with the detection of the electrode 13 starts a timer circuit 17 through a pulse amplifier 16 and the signal of the electrode 14 comes to a stop signal stopping the timer circuit 17 through a pulse amplifier 18. This timer operation time can be shown by a formula T=C1√m+C2 (wherein m is ion mass and C1 and C2 are constant determined by an apparatus) and ion mass can be calculated. That is, a measuring time in the timer 17 is outputted as mass spectrum vector data by a computer 19. Thereby the practicality is enhanced.