PROBLEM TO BE SOLVED: To provide the delay time difference measurement method by which the delay time is measured with high accuracy. SOLUTION: A time difference between a signal sent through a 1st transmission path 34 with a 1st measured object 40 inserted thereto and a signal sent through a 2nd transmission path 36 with a 2nd measured object 42 inserted thereto is measured as a 1st delay time difference Δt1. The 1st measured object 40 and the 2nd measured object 42 are replaced with each other, and a time difference between a signal sent through the 1st transmission path 34 with the 2nd measured object 42 inserted thereto and a signal sent through the 2nd transmission path 36 with the 1st measured object 40 inserted thereto is measured as a 2nd delay time difference Δt2. Then the delay time difference between the 1st measured object 40 and the 2nd measured object 42 is obtained as a half of the difference between the 1st delay time difference Δt1 and the 2nd delay time difference Δt2.
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