The invention relates to a filter device (1) having a filter layer (2) extending along two main extension directions, comprising electrically non-conductive filter material (3). A fluid can flow through the filter material (3) transversely to the two main extension directions in order to filter substances out of the fluid. A microwave resonator (5) is formed using electrically conductive, surface-running conductor material (4) linearly spanned along one of the two main extension directions. The conductor material (4) delimits a part of the filter layer (2) made of the filter material (3), which extends in the form of a strip in the surface spanned by the two main extension directions, at least on the broad side. An electromagnetic connection (9, 10) is provided for exciting a microwave in the microwave resonator (5) and for detecting the excited microwave in the microwave resonator (5).