A shape measurement method comprising: a step of irradiating a measurement object with an optical pulse train in which a plurality of optical pulses (CP, CP,..., CP(k)) that have predetermined frequency distributions in which frequency changes in a predetermined pattern on a time axis are disposed chronologically in numerical order to associate each optical pulse (CP, CP,..., CP(k)) with the corresponding number (1, 2,..., k) in the numerical order; and a step of measuring an optical shape of the measurement object in accordance with a shift of the numbers (1,2,..., k) in the numerical order of the optical pulses (CP, CP,..., CP(k)) of a plurality of detection target optical pulse trains (B1, B2,..., Bn) after the emitted optical pulse train acts on the measurement object and a difference between the frequencies in the optical pulses (CP, CP,..., CP(k)) of the plurality of detection target optical pulse trains (B1, B2,..., Bn), wherein the frequency of the plurality of optical pulses (CP, CP,..., CP(k)) is a function of the number in the numerical order and a repetition frequency of the optical pulse train, wherein in the step of measuring the optical shape of the measurement object, the plurality of detection target optical pulse trains (B1, B2,..., Bn) are instantaneously cut on a predetermined plane in a measurement region at a timing depending on a reference optical pulse train being derived from the optical pulse train and propagating along a predetermined optical path without reaching the measurement object, wherein the shift of the numbers in the numerical order is acquired by applying the optical pulse trains having different repetition frequencies, obtaining a rate of changes in the frequency of the detection target optical pulse trains (B1, B2,..., Bn) cut on the predetermined plane, and determining the shift of the numbers in the numerical order in accordance with a predetermined relation between the rate of changes in the frequency of the detection target optical pulse trains (B1, B2,..., Bn) and the number in the numerical order, and wherein the difference between the frequencies in the optical pulses (CP, CP,..., CP(k)) is acquired from the detection target optical pulse trains (B1, B2,..., Bn) cut on the predetermined plane. a step of irradiating a measurement object with an optical pulse train in which a plurality of optical pulses (CP, CP,..., CP(k)) that have predetermined frequency distributions in which frequency changes in a predetermined pattern on a time axis are disposed chronologically in numerical order to associate each optical pulse (CP, CP,..., CP(k)) with the corresponding number (1, 2,..., k) in the numerical order; and a step of measuring an optical shape of the measurement object in accordance with a shift of the numbers (1,2,..., k) in the numerical order of the optical pulses (CP, CP,..., CP(k)) of a plurality of detection target optical pulse trains (B1, B2,..., Bn) after the emitted optical pulse train acts on the measurement object and a difference between the frequencies in the optical pulses (CP, CP,..., CP(k)) of the plurality of detection target optical pulse trains (B1, B2,..., Bn), wherein the frequency of the plurality of optical pulses (CP, CP,..., CP(k)) is a function of the number in the numerical order and a repetition frequency of the optical pulse train, wherein in the step of measuring the optical shape of the measurement object, the plurality of detection target optical pulse trains (B1, B2,..., Bn) are instantaneously cut on a predetermined plane in a measurement region at a timing depending on a reference optical pulse train being derived from the optical pulse train and propagating along a predetermined optical path without reaching the measurement object, wherein the shift of the numbers in the numerical order is acquired by applying the optical pulse trains having different repetition frequencies, obtaining a rate of changes in the frequency of the detection target optical pulse trains (B1, B2,..., Bn) cut on the predetermined plane, and determining the shift of the numbers in the numerical order in accordance with a predetermined relation between the rate of changes in the frequency of the detection target optical pulse trains (B1, B2,..., Bn) and the number in the numerical order, and wherein the difference between the frequencies in the optical pulses (CP, CP,..., CP(k)) is acquired from the detection target optical pulse trains (B1, B2,..., Bn) cut on the predetermined plane.