In Japan, few reports have discussed adverse events and safety after simultaneous vaccination during infancy. The purpose of this study was to elucidate adverse events after simultaneous vaccination of inactivated vaccines in infants in comparison with those after single vaccination. Selected for this study were infants aged ≥2 months who received subcutaneous injections of inactivated vaccines between July 2012 and June 2013. These subjects were divided into two groups: a single-vaccination group (46 subjects) and simultaneous-vaccination group (42 subjects). The presence or absence of severe adverse events that required hospitalization was investigated. We also checked up on subject background and systemic [fever (transitional and highest body temperature)] and local (dermatological, respiratory, gastrointestinal, neurological, and other organs’ symptoms) adverse events. Questionnaires to investigate if adverse events occurred during one week after simultaneous vaccination, questionnaires were distributed to the parents of all the subjects. “We performed vaccination in 162 subjects, and the collect rate of questionnaires was 57% (97/162).” The percentage of effective answers to the questionnaire survey was 91% (88/97). Among simultaneous-vaccination group subjects, 14 (32%) received Haemophilus influenzae type b (Hib) + 7-valent pneumococcal vaccine (PCV7) and 12 (27%) received Hib + PCV7 + Diphtheria-Pertussis-Tetanus vaccine. No subject developed severe adverse events that required hospitalization. The body temperatures taken on the day following the injection in the simultaneous-vaccination group were significantly higher than those in the single-vaccination group (p=0.049). However, the incidence of other systemic and local adverse events in the simultaneous-vaccination group was not significantly different from that in the single-vaccination group. Compared with single vaccination, simultaneous vaccination of inactivated vaccines in infants resulted in a significant rise in body temperature on the day following vaccination; however, no severe adverse events occurred.