A filter device has a filter housing part (10) with a filter element (12) and can be detachably connected to another filter housing part (20). A valve (24) clears a fluid path in the interconnected state of the housing parts (10, 20) with the filter element, and in the separated state of the housing parts (10, 20) from one another or without the filter element at least partially blocks it. Instead of known quarter-turn fastener and stopper solutions, a blocking is devised which can be economically implemented and which requires decidedly less installation space. A process for operation of a filter device is also provided.