A multimode light generating module comprises an optical cavity and a light-receiving element. The optical cavity is constituted by a semiconductor optical amplification element, and an optical fiber having a grating. The optical fiber comprises a grating having an apodized periodic refractive index distribution, and the slope of the reflection spectrum in a longer wavelength region with respect to the peak wavelength _p is gentler than that in the shorter wavelength region therewith. The grating satisfies the relationship of 0.5 nm_0.2-_p0.1 nm. Here, _0.2 is defined by _0.2^h-_0.2^l, where _0.2^h is the longer one of two wavelengths exhibiting a relative reflectivity of 0.2 with respect to the maximum reflectivity, and _0.2^l is the shorter one of two wavelengths exhibiting a relative reflectivity of 0.2 with respect to the maximum reflectivity.