PURPOSE: To easily wash a test tank with an inexpensive apparatus by dripping a waterdrop on a part of the wall of a locally heated container or the heating plate arranged in the container and increasing or decreasing the amount thereof.

CONSTITUTION: A water storage tank 10 is fixed to the intermediate space of a test tank 100 and water 18 is received in the tank 10. A waterdrop falls to the point H of the bottom part of the test tank 100 through the dripping hole 16 provided at the bottom part of the water storage tank 10 by raising the valve support rod 14 connected to a valve 12 for a definite time. The point H is preheated by a heater 24 and the falling water 18 immediately begins to evaporate and the inner pressure of the test tank 10 detected by a pressure sensor 20 rises. When this pressure does not reach the set value of a pressure control meter, the same operation in repeatedly continued until the pressure reaches the set value. Since the generation of unsaturated steam is performed on a heating plate by this constitution, a double tank structure is unnecessary and, since the pressure of steam is controlled on the basis of the amount of intermittently dripping water, a structure is made simple and washing is made easy.