To realize selective detection of spectrum within a specified range by providing a plurality of dispersion gratings for producing a spectrally dispersed beam.

A dispersion unit comprises a plurality of dispersion gratings having surface for forming a spectrally dispersed beam. Light 17 from a radiation source 11 passes through an inlet slit 16 and reflected on a collimator 18 toward a reflective dispersion grating 20 having saw-tooth grooves 21. An initial spectrum 23 is directed toward a step grating 22 having dispersion lines crossing the grating 20. The radiation is further reflected from the grating 22 to produce another dispersion beam 24 which passes through a reflector 26 and focused on a detector 34 through a mirror 28 and a lens 30. The detector 34 senses the radiation impinging thereon and produces a signal which is delivered on a line 35 toward a computer station 14. The computer processes the signal information and displays the results in the form of concentration of atomic element in the sample.