An ultrashort light pulse oscillated from an ultrashort pulse oscillator enters a waveguide (2) via a polarization control element (3). After conversion into a supercontinuum by a nonlinear optical effect, it is compressed by a prism pair compressor (71) as pulse compressor (7), and then emitted. The waveguide (2), which is a nonlinear fiber with normal dispersion in the wavelength range from 850 to 1550, generates the supercontinuum having a spectrum continuous in a wavelength band width of at least 200 nm included in the wavelength range from 850 to 1550 nm. The supercontinuum, which has a peak power within 1 to 100 kW, can be used as excitation light in a multiphoton excitation fluorescence microscope for fluorescence observation of biological samples.