To provide a wiring harness duct structure which can prevent rain water, etc., flowing along a wiring harness from penetrating into a cargo compartment at a low cost.

In order to provide moisture-proofness of insertion parts 19, through which a wiring harness 18 enters and comes out of a wiring harness laying space 3A defined by the floor member 2 of the passenger compartment of a bus and the ceiling plate 6 of a cargo compartment 3 provided beneath the floor member 2, harness ducts 20 which protrude from the inside to the outside of the space 3 and partially cover the wiring harness 18. Furthermore, the ducts have slopes with gradients upward from the outside to the inside of the space 3A are provided in the insertion parts 19 respectively. The respective ducts 20 are filled with a sealing agent 24 for blocking the gaps formed around the wiring harness 18 in the insertion parts 19.