A one-way valve for a fuel-tank filling conduit deters siphoning and prevents back flow of fuel. The one-way valve comprises a blocking element arranged in the conduit and pivotable between a flow-blocking, object-insertion blocking first position and a flow-admitting, object-insertion blocking second position. In the second position the blocking element divides the conduit into flow areas of comparable width. The one-way valve further comprises a bias element that biases the blocking element to the first position.