Disclosed is a wire harness-routing device which enables a large protector-containing space in a rail to be secured to enable a large number of electric wires to be routed. The wire harness-routing device (1) is provided with: two wire harnesses (10); a support body (3) for supporting a seat and slidably attached to the rail (2); and a protector (4) sliding in association with the movement of the support body (3) and guiding the two wire harnesses (10) to the seat. The protector (4) is provided with: a pair of wire harness-holding sections (43) for holding the two wire harnesses (10) which are led to the inside of the rail (2) from an opening (20), the wire harness-holding sections (43) holding the two wire harnesses (10) in such a manner that the two wire harnesses (10) are arranged parallel to each other; a led-out section disposed between the pair of wire harness-holding sections (43) and extending in a tube-like shape, the led-out section being, after being passed through a slit (21), led out to the outside of the rail (2) and having the two wire harnesses (10) arranged therein and passing therethrough; and a mounting section (68) mounted to the support body (3) with play which allows the mounting section (68) to move in the widthwise direction of the rail (2).