A method and an apparatus for measuring the concentrations of the components of a fluid are described, which can be used to measure the concentrations of the components continuously in real time and to monitor a high pressure gas, and is suitably used for in-line monitoring. In the method and the apparatus, a fluid sample is conducted through a measuring tube, wherein the measuring tube has a small aperture with a constant diameter in a fluid flow path. The pressure difference (P1-P2) between the upstream and the downstream of the small aperture and the flow rate at the downstream of the small aperture are measured to determine the concentrations of the components of the fluid.