Water level sensor and heat exchanger sterilizing and humidifying apparatus having the same

A water level sensor is closely adhered to a storage tank, and a heat exchanger sterilizing and humidifying apparatus allows the water level sensor to be mounted at the outside of the storage tank, thus having a simple configuration. The water level sensor may include a holder, a cap connected to the holder so as to reciprocate within the holder, a case connected to the cap so as to reciprocate within the holder, a sensor part attached to the case to sense the water level in the storage tank, and a spring located between the holder and the cap, wherein the sensor part mounted at the outside of the storage tank and attached to the case by restoring force of the spring when the storage tank contacts the case may be closely adhered to the storage tank and thus may sense a level of water stored in the storage tank.