The present invention provides a method for narrowing a spectral width that can also be adapted to an ultrashort optical pulse emitted from a wavelength tunable light source, and that can provide an output optical pulse with a narrow spectral width and a low noise component, and an optical element and a light source device that use the method for narrowing a spectral width. The method includes using an optical waveguide member (2) to cause a soliton effect in an input optical pulse (1) within the optical waveguide member (2), thereby narrowing a spectral width of the input optical pulse (1) to provide an output optical pulse (3), the optical waveguide member (2) having dispersion characteristics such that the average of a second-order dispersion value (β2) with respect to the input optical pulse (1) is negative, and the absolute value of the second-order dispersion value (β2) increases in a propagation direction of the input optical pulse (1).