Abstract: The ion optics system of the CH_4 type mass spectrometer is calculated byusing of matrix method and program TRANSPORT in this paper. A proposalfor improving the system is pressnted. It is shown that the ion beam can befocused with second order in horizontal direction and first order in verticaldirectiou, if magnetic shimming is made both at the entrance aud exit of theanalyzer. In such a case the resolving power and the sensitivity of the spec-trometer can be improved considerably.