Light beams swept in wavelength repeatedly within first and second wavelength ranges respectively are outputted from a light source unit at the same time. Each light beam is split into measuring and reference beams. Reflected beams from a measuring object when the measuring beams are irradiated on the measuring object are combined with the reference beams. The wavelengths of the interference beams produced thereby are divided into three wavelength ranges, for example, a range not greater than 0.9 μm, a range from 0.9 to 1.2 μm, and a range longer than 1.2 μm by a wavelength dividing means to obtain interference signals. In the wavelength range from 0.9 to 1.2 μm which includes an overlapping wavelength range where the wavelength ranges of the light beams overlap with each other, either one of the light beams is outputted from the light source unit.