A sample tube is used to ensure uniformity in a static magnetic field and uniformity in electromagnetic wave irradiation for NMR measurement for continuous sample injection. The sample tube is formed of a signal detecting tube having a length lying between 80% and 100% of the length of an antenna, the signal detecting tube accommodating a sample at the position of the antenna; first and second joint tubes each having an outside diameter equal to the outside diameter of the signal detecting tube and having an inside diameter smaller than the inside diameter of the signal detecting tube; and injection and ejection supporting tubes each having an inside diameter smaller than the inside diameter of the signal detecting tube. The first and second joint tubes have magnetic susceptibility matched to or brought close to that of a sample solvent.