PURPOSE: To accurately control the water level of a dishwasher by a method wherein the slopes of lines expressing water level changes per unit time at the start of water feed and at the end of water feed are both obtained and the water level at the intersection of the extensions of these lines is made to be the reference water level.

CONSTITUTION: A water level detecting means 6 is for converting a change in the water level into a change in the frequency and, as the water level rises, the frequency is lowered. Since a tank 2 is of concave shape, this tank has a characteristic that the level of water being fed rapidly rise to a certain water level from zero and later change in the water level slowly. From a graph of oscillation frequency (f) versus water feed time (t), the slope Δf1 of the oscillation frequency upon the initial of water feed and the slope Δf2 thereof at the end of water feed are obtained, the frequency of f1 is obtained from the intersection of the extensions of these curves and the reference water level is then obtained to measure the water level at the end of water feed. Thus, the subject water level control device permits the optimum operating time to be set according to the water feed amount and the target water level to be accurately attained all the time.