A device (22) for measuring unburned carbon in a sample of ash-containing flue gas from combustion of carbonaceous fuel, the device (22) being designed with an ash sampling arrangement (19) and a sample compartment (8) extending through a microwave resonancechamber (7) which is connected to a microwave generator (15) and a microwave receiver (20), the sampling chamber (8) being provided with valves (4, 12) at inletand outlet, and a level meter (6), and wherein a pressurised air connection (5) for the samplechamber (8) is provided at a position between the inlet valve (4) and the microwave resonance chamber (7), the pressurised air connection (5) being connected with a source of pressurised air (21). A method for measuring unburned carbon by taking a sampleofash-containing flue gas from combustion of carbonaceous fuel, wherein microwave resonance measurement is performed on the sample at a first and a second degree of compression given by the pressure in the sample chamber (8), and where the measured values in combination are compared with a reference curve for determining the residual carbon content in the sample.