A first tunable wavelength pulse light source (22) is driven by a reference signal to emit a first optical pulse. An optical demultiplexer (24) demultiplexes a first optical pulse emitted from the first pulse light source (22) into a reference optical pulse and an incident optical pulse to be sent into an object to be measured. An optical multiplexer (26) multiplexes the reference optical pulse and an outgoing optical pulse passing through the object to output multiplexed light. A second pulse light source (23) generates a second optical pulse which is synchronous with the first optical pulse and delays a predetermined time for each period of the first optical pulse. A sampling unit (27, 27a, 27b) receives the multiplexed light and the second optical pulse to obtain an optical pulse train signal proportional to the intensity of the multiplexed light obtained in synchronism with the second optical pulse. From the optical pulse train signal from the sampling unit (27, 27a, 27b), a signal processor (37) obtains an envelope formed by peaks of individual optical pulses forming the optical pulse train. The wavelength dispersion of the object is obtained by measuring the delay time of the outgoing optical pulse passing through the object on the basis of intervals between the peaks of the envelope.