The silicone oil having nitrogen atoms in the side chains and used for treating the silica fine powder has at least the partial structure having the following formula: ##STR6## wherein R7 is a hydrogen atom, an alkyl group, an aryl group or an alkoxy group; R8 is an alkylene group or a phenyl group; R9 and R10 may be the same as or different from each other and are each a hydrogen atom, an alkyl g