An aminoglycoside compound of the following formula ##STR1## wherein one of R.sub.1 and R.sub.2 represents a hydrogen atom and the other, a methyl group, R.sub.3 represents a hydrogen atom or an amino acyl group having 2 to 5 carbon atoms, R.sub.4 represents a lower alkyl group substituted by one or two substituents selected from the class consisting of hydroxy and amino groups, or a hexopyranosyl group whose hydroxy groups may be substituted by amino groups,and a pharmaceutically acceptable acid addition salt thereof; and an antibiotic composition comprising said compound and its pharmaceutically acceptable acid addition salt.