As the first step to develop a liquid chromatography/multiple-stage mass spectrometry (LC/MSⁿ) analytical system which allows us to simultaneously screen relevant drugs and poisons in forensic specimens, an efficient and practical analytical method for correcting LC retention data has been produced utilizing a liquid chromatography/mass spectrometry (LC/MS). The method is based on retention indices (RI). LC/MS employed a C18 (ODS) semi-micro column with a methanol gradient elution program, and the RI values of target substances were calculated on the basis of the retention times of a substance and tri-n-alkylamines, which were chosen as reference markers (internal standards) and respectively assigned RI values. The present study has indicated that RI provides a very reproducible method for correction of retention data. This system has also been found to be a potentially important tool in simultaneously correcting LC retention data of target substances even when we modify the LC conditions such as the solvent-gradient and flow rate. This method will, thus, be successfully incorporated into routine work for forensic investigation.