an embodiment of the invention will hereunder be described with reference to the accompanying drawings . fig1 is a schematic diagram of an excitation wavelength sweeping type raman spectroscopic apparatus of the invention . the spectroscopic apparatus includes a wavelength variable laser source 1 , a laser beam converging system 5 , a scattered light converging system 7 , a bandpass filter 8 , and converging system 9 , a photodetector 10 , and a signal processor 11 . a part of an output beam from the wavelength variable laser source 1 is split by a beam splitter 2 and guided to a laser wavemeter 3 and a bandpass filter 4 . the laser wavemeter 3 may be used when an oscillation wavelength of the wavelength variable laser source 1 is to be measured correctly . also , the bandpass filter 4 may be used when light that gives measurement background is contained in the wavelength variable laser beams , and noise can thereby be removed . the band pass filter 4 will be described in detail with reference to fig3 . in fig3 reference numeral 21 designates a dye laser device which corresponds to the wavelength variable laser source 1 shown in fig1 . the dye laser device 21 employs nitrogen laser light or yttrium aluminum garnet ( yag ) laser light as excited light , and is able to output a laser light beam having a different wavelength . the bandpass filter 4 in fig1 includes a diffraction grating 22 , a driver 23 , a mirror 24 , a lens 25 , a beam splitter 26 , a pinhole section 27 , a lens 28 , a one - dimensional photodetector 29 and a controller 30 , as shown in fig3 . the laser light beam having a given wavelength output from the dye laser device 21 is diffracted by the diffraction grating 22 and goes to the mirror 24 . the laser beam 31 output from the mirror 24 is converged at the pinhole of the pinhole section 27 by the lens 25 , and the components other than the laser beam are removed by the pinhole . next , the laser beam is collimated by the lens 28 to produce a parallel light beam , which is used as an excited light beam for raman measurement . on the other hand , the laser beam 31 which passed through the lens 25 is split by the beam splitter 26 , and part of the split beam is irradiated onto the one - dimensional photodetector 29 . when the wavelength of the light beam from the dye laser device 21 is varied , the diffraction angle of the diffraction grating 22 is varied , so that the path of the laser beam is changed to the position indicated by a broken line 32 . as a result , since the position of the laser beam which is detected by the one - dimensional detector 29 is varied , the set angle of the diffraction grating 22 is controlled such that the position of the laser beam at the detector 29 is fixed . that is , the driver 23 is controlled by the controller 30 to change the set angle of the diffraction grating 22 , so that the laser beam always passes through the pinhole of the pinhole section 27 . thus , when the wavelength of the laser beam output from the dye laser device 21 is swept , the passing wavelength of the bandpass filter 4 can be varied . further , since the set angle of the diffraction grating 22 is varied in accordance with the laser wavelength , the wavelength range of passing can be broadened extremely . the excited laser beam from the bandpass filter 4 irradiates a specimen 6 while being converged by the converging system 5 . this causes the raman scattered light to be radiated efficiently from the specimen 6 . the scattered light is converted by the scattered light converging system 7 and the converged light passes through the bandpass filter 8 . at this point , the bandpass filter 8 selects only a beam of light of a predetermined wavelength and allows the selected beam to pass therethrough . the beam of light selected by the bandpass filter 8 reaches the photodetector 10 after being converged by the converging system 9 . a signal from the photodetector 10 is processed by the signal processor 11 . in the above case , a raman spectrum of the specimen 6 can be recorded by recording the intensity of a signal from the photodetector 10 while sweeping the oscillation wavelength of the output beam from the wavelength variable laser source 1 . also , the raman shift can be determined from both the oscillation wavelength of the output beam from the wavelength variable laser source 1 and the central wavelength of the bandpass filter 8 . the resolution of the raman spectroscopic apparatus of the invention is determined by the linewidth of the excited laser beam and the characteristics of the bandpass filter . however , since the linewidth of the excited laser beam can be narrowed sufficiently , bandpass filters of different characteristics may be used by additionally providing a mechanism for changing one bandpass filter with another . this will make the resolution of the raman spectroscopic apparatus variable . further , if the range of varying the wavelength of the wavelength variable laser beam is so narrow that measurements over a wide range cannot be made , a mechanism for changing bandpass filters of different central frequencies may be additionally provided , so that the measurable range can be increased . the excitation wavelength sweeping type raman spectroscopic apparatus of the invention has the following advantages compared with the conventional raman spectroscopic apparatuses . since the bandpass filter is bright and has a high transparency , the raman scattered light can be detected efficiently . further , since the bandpass filter is small , light , and inexpensive , a small , light and inexpensive raman spectroscopic apparatus can be fabricated . the absence of an entering slit allows the raman scattered light from a wide region to be detected efficiently . even if the resolution is improved by narrowing the linewidth of an excited laser beam , the bandpass filter , whose transmittance is not impaired compared with a spectroscope , can be used for measurements at high resolutions . since the wavelength of scattered light to be detected is constant , the optical systems such as the converging system can be designed easily . in addition , the raman shift can be determined correctly by measuring the wavelength of an excited laser beam . since the wavelength of scattered light to be detected is constant , the sensitivity of the detecting system remains unchanged . therefore , the sensitivity of the raman spectrum can be corrected easily by measuring the output of a laser source .