The liquid level sensor comprising a tubular case with an inner surface, a base plate having a wiring and disposed in the case so as to extend lengthwise of the case is disclosed. A switch member is disposed on the base plate and is electrically connected with the wiring. A float is disposed outside of the case and is movable in accordance with a displacement of the liquid surface of the liquid in the reservoir tank. A driving member is disposed on the float so as to drive the switch member. A retaining member is disposed on at least one of the inner surface of the case and the base plate so as to support and secure the base plate in the case.