In the present work, a multipass optical system intended for increasing the sensitivity of a Raman gas spectrometer based on the 90° geometry of scattered light collection is described. The system is characterized by an adjustment stability and an increased number of laser beams that pass through a small scattering volume, thus allowing the intensities of Raman signals from components of the gas medium in this volume to be increased. It is demonstrated that the application of this multipass optical system allows the sensitivity of the Raman gas spectrometer to be increased practically by 20 times (to several ppm for the 30-s registration time).