An NMR (nuclear magnetic resonance) probe is offered which produces a magnetic field strength adapted for producing an NMR signal within a sample coil. The probe has a resonant circuit including a sample coil, a first transmission line, and a second transmission line. The resonant circuit is designed taking account of the transmission line length L.sub.s of the coil, the transmission line length L.sub.a of the first line, and the transmission line length L.sub.b of the second line. The coil is placed in a position where the magnetic field strength is maximized. Where the electrical characteristics of the resonant circuit are varied by connection of the coil and transmission lines, the transmission line lengths are corrected according to the variations.