PURPOSE: To enable a holding of a water level of a cooling water in view of its numerical value in a fine mode by a method wherein an ultrasonic wave reflecting float is arranged on a cooling water surface within a main body of a tank, and an ultrasonic wave transmitting head, an ultrasonic wave receiving head and a convertor for calculating and displaying a distance from the received ultrasonic wave to the float are fixed and mounted above the float.

CONSTITUTION: A float 35 is moved up and down in response to an amount of cooling water within a main body 11 of a tank so as to always indicate its water level. Ultrasonic wave transmitted from a transmitting head 36a is reflected at an upper surface of the float 35 and received by a receiving head 36b. This receiving signal is sent to a converter 37, a distance up to the float 35 is calculated in response to a required time from a transmitting of the ultrasonic wave to its receiving at the converter 37, this is converted into a position of the float 35, i.e. the water level of the cooling water and then it is transmitted as an output signal. The aforesaid water level is displayed by the output signal. Since the water level of the cooling water is displayed numerically from time to time, the water amount of the cooling water and its variation can be grasped by a level detector 3 in a fine mode and thus a positive accommodation is possible even under an abnormal state such as a rapid reduction of the water volume.