In the paper it is reported that using a prism-fiber coupler, the propagation coefficient βmn for scalar modes travelling in optical fiber can be determined by measuring coupling angles or positions of m-lines. Each scalar mode can be excited by varying the coupling angle. The so-called m-line corresponding to each scalar mode is coupled out of an output prism. Its characteristics are different from those of the m-line coupled out of a planar waveguide. The degeneration of βmn is shown on the m-line photograph. A good agreement is obtained between, the measured and calculated values of βmn.