Abstract: A leak-free drinking beaker comprises a container as well as a lid closing the container, a drink opening which opens into the interior of the container, and a valve extending over the drink opening, which valve is held in the closed position under a certain pretension and can be released against said pretension under the influence of a pressure difference. The valve is a double-acting non-return valve and comprises a flexible valve element and two separate valve seats, the valve element and one of the valve seats has a blocking action in a direction opposed to the direction in which the valve element has a blocking action with the other valve seat.