To provide a water level sensor capable of preventing an oil, an impurity and a dust from being stuck on a sensor body section such as a sensor case or a float and of accurately detecting a water level even when a water surface in a tank is waving.

This device consists of a sensor body and an external case 11 housing the sensor body. The sensor body comprises at least switches 5, 6 that operates in response to magnetism and a float 2 with a magnet 8 attached thereto which is placed in the vicinity of the switches 5, 6 and moves vertically along with the variation of the water level. The external case 11 is equipped with a water injecting nozzle 12 and an air drain hole 15 placed on the top end which is not submerged below the water surface. It is also equipped with an inlet/outlet hole 13 placed on the lower end portion of the case which is submerged below the water surface. A water scattering plate 14 with a water scattering hole 14a thereon is provided to the lower part of the water injecting nozzle 12.