Exploratory Clinical Study of MT-2301

Inclusion Criteria: * Healthy infants aged ≥2 and \<7 months at the first vaccination of the study drug * Written informed consent is obtained from a legal guardian (parent) Exclusion Criteria: * With obvious pyrexia (axillary temperature of 37.5ºC or higher) at vaccination of the study drug * With known serious acute disease * With known underlying disease such as cardiovascular disease, renal disease, hepatic disease, blood dyscrasia, and respiratory disease * With past diagnosis of immunodeficiency or currently under immunosuppressive treatment * History of anaphylaxis due to food or pharmaceuticals * With experience of Hib infection, diphtheria, pertussis, tetanus, and acute poliomyelitis * With experience of Hib vaccination, or administration of vaccine including either diphtheria, pertussis, tetanus, or polio as a constituent * History of convulsions * Administered a live vaccine within 27 days before the first vaccination of the study drug, or inactivated vaccine or toxoid within 6 days before vaccination * Administered transfusion, immunosuppressant (excluding drugs for external use), or immunoglobulin formulation * Administered corticosteroid 2 mg/kg per day or more as prednisolone (excluding drugs for external use) continuously for more than 1 week * Participated in other studies within 12 weeks before obtaining consent