Abstract: Microwaves are respectively propagated through a reference fluid and fluid to be measured, both contained in a detection pipe body, and as a result, a first phase lag &thgr;1 and a second phase lag &thgr;2 can be determined on the basis of the microwaves by a phase lag measuring circuit. Then, a density computing circuit subtracts the first phase lag &thgr;1 from the second phase lag &thgr;2 to determine a phase difference &Dgr;&thgr;. When computing the density of matter in the fluid on the basis of the phase difference &Dgr;&thgr;, the density computing circuit performs density computing processing such that a real second phase lag &thgr;2 satisfies the following equation &thgr;2=&thgr;2′+N×360° where N is the number of rotations, and &thgr;2′ is an apparent phase lag obtained by the phase lag measuring circuit.