A new spectral measurement technique is provided which enables measurement even if the light to be measured exists for a very short period. In one embodiment, a broadband pulsed light wave whose wavelength shifts temporally and continuously in a pulse interferes with a light wave to be measured. The intensity at each wavelength of the light wave to be measured is obtained using a Fourier transform of the output signal from a detector that has detected the intensity of the wave resulting from the interference. A laser beam from a laser source is converted to a supercontinuum wave by a nonlinear optical element, and a pulse extension element extends pulses of the supercontinuum wave, thus generating the broadband pulsed light wave.