In this study, we examined the accuracy and rapidity of drug susceptibility determination using clinical isolates of MRSA with the fully automated rapid identification susceptibility testing system RAISUS S4. Ninety eight MRSA strains were used and the time until methicillin resistance was determined was analyzed by both of the standard method (18-hr method) and the rapid method. Five strains (5.1%) were determined to be methicillin-sensitive in MPIPC by rapid method only while all strains were determined to be resistant in CFX. The average methicillin resistance determination time was 7.0/5.0 hr for MPIPC, 6.3/5.0 hr for CFX, and 6.3/5.0 hr for the combination of MPIPC and CFX by the standard/rapid method, with the rapid method being significantly shorter (Wilcoxon's signed rank sum test, p<0.01). Strains determined to be methicillin-sensitive by MPIPC tended to have a longer time to methicillin resistance by the standard method, but this effect was much less pronounced for the rapid method using CFX. Methicillin resistance determination by the rapid method using RAISUS S4 enables rapid detection of MRSA without false-susceptible results, which may lead to early and appropriate treatment.