This soundproof structural body has: a first pipe structure; and a second pipe structure that is connected to the first pipe structure and that has a different cross-sectional area size from the first pipe structure, wherein a structural body having a cross-sectional area smaller than that of the first pipe structure is installed inside the first pipe structure, and the transmission loss, obtained when the structural body is installed inside the first pipe structure as compared to when the structural body is not installed inside the first pipe structure, is more unlikely to exhibit a columnar resonance mode and is positive at two adjacent frequencies. This soundproof structural body exhibits a soundproof effect even at a frequency other than that of columnar resonance of a pipe structure such as a duct or a muffler, is compact, and can provide a high transmission loss in a wide band.