PURPOSES : This study is to analyze expected effect of a accident decrease when motorcycle safety inspection is introduced. METHODS : Based on the literature review of effect of 4-wheel vehicle inspection, probability of occurring accidents among defective motorcycles are calculated by using the number of estimated defective motorcycles and accidents resulting from defects of motorcycles. Then, the number of decreased accidents which is resulting from eliminating defects of motorcycles by safety inspection is estimated by using probability of occurring accidents among defective motorcycles. RESULTS : If the ratio of eliminating defects of motorcycles is 95% after motorcycle safety inspection, the effects of accident decrease of motorcycle safety inspection are analyzed from 2005 to 2008. As a result, 46,292 defective motorcycles are repaired and 1,376 accidents are prevented when the probability of occurring accidents among defective motorcycles is 2.97%. CONCLUSIONS : This study suggests the expected effect of motorcycle safety inspection is that the inspection can prevent 1,376 motorcycle accidents. However, the number of preventing motorcycle accidents are small, compared with the total number of motorcycle accidents because there are limitations to investigate the causes of defective motorcycle accidents. A more precise analysis of the expected effect of motorcycle inspection is possible when a systematic investigation of the causes of the accidents is implemented.