To provide an elbow with built-in straightening guide vanes achieving a uniform flow with identical flow velocity distribution after passing through the straightening guide vanes, and a cavitation tunnel having the elbow with the built-in straightening guide vanes.

Each of straightening guide vanes 23 is divided into three blocks 23A, 23B, 23C each having a mutually equal attack angle. The straightening guide vanes 23 in the blocks 23A, 23B, 23C are arranged so that the straightening guide vanes 23 in the outside block 23C have a smaller attack angle and the straightening guide vanes 23 in the inside block 23A have a larger attack angle with the attack angle of the straightening guide vanes 23 in the block 23B arranged in a center, as a criterion.