One-way valve, especially low-pressure check valve, for use in the medical technology, having a housing consisting of an inlet half and an outlet half and having an inlet channel and an outlet channel with a diaphragm being positioned in a pressure space which is contacting an annular valve seat under pre-tension which is opening into the outlet channel. The pressure space is consisting of two pressure chambers. In the direction of flow before the closing mechanism formed by the valve seat and the diaphragm a second valve mechanism is provided which is opening in the same sense with the first closing mechanism at an over-pressure in the entry channel and which is closing at an over-pressure in the exit channel.