A water level measuring apparatus and a water level measuring method, in which a water level L0 of a liquid is read from a water level indicator secured to a liquid tank, and a filter value FLT is calculated by applying a filter with a time constant greater than one second to the water level L0, and the filter value FLT is output as a final water level L. Accordingly, even if the surface sloshes and the water level L0 fluctuates up and down, the water level L0 is smoothed by the filter. Hence there is little influence of the liquid sloshing, and the water level measurement accuracy can be improved.