A rolling-bearing sealing device includes: an outer ring fixed to a rotating sprocket housing; an inner ring fixed to a non-rotating motor housing; a plurality of rolling elements between the outer and the inner rings; a retainer holding the rolling elements; and an oil seal including a slinger and a seal lip and fixed to the inner ring, in which a radial lip of the oil seal is pressed against the outer ring or the sprocket housing, and in which the sprocket housing has an end face provided with a side plate fixed thereto, for sliding contact by a side lip extended axially from the oil seal.