In order to start sweeping an optical spectrum analyzer 1 and a wavelength tunable light source 14 on the same timing, a motor 6 for driving a spectroscope 4 in the optical spectrum analyzer 1 is controlled. To this end, a drive circuit 7 outputs a control signal to determine the timing where the motor 6 starts to rotate. A wavelength control circuit 19 in the wavelength tunable light source 14 controls the sweep of the wavelength of single-mode oscillation from a light source unit 20 and starts sweeping the light source unit 20 in response to a signal externally supplied to control the timing of sweep start. As a result, the sweep of the wavelength being measured with the optical spectrum analyzer 1 and that of the wavelength of single-mode oscillation from the wavelength tunable light source 14 are started on the same timing and high-speed sweep is achieved.