To provide a drain trap which can simplify maintenance operations by allowing effluent to flow to a discharge unit properly. The drain trap has a main body equipped with a communicating path which communicates a receiving unit and a discharge unit; a storage unit which, being installed in the communicating path, stores effluent received by the receiving unit; and a normally-closed check valve which, being installed in the communicating path, floats up to allow the effluent in the storage unit to flow to the discharge unit when the effluent reaches or exceeds a set amount, in which the storage unit is installed in such a way as to communicate with a part right under the normally-closed check valve, and the discharge unit is installed in the lowest part of a discharge-side communicating path so that no effluent will be stored in the discharge-side communicating path, which is that part of the communicating path which runs between the normally-closed check valve and the discharge unit.