A moving inspection device capable of realizing significant size reduction/weight reduction without affecting the inspection performance for an inspection target, a moving inspection method, and a method for manufacturing a steel material. A moving inspection device includes: a moving inspection device body configured to inspect an inspection target for defects while moving over a surface of the inspection target; and water supply devices separate from the body and configured to supply water required for the inspection onto the surface of the inspection target. The body is installed with a flow adjustment plate configured to push out the water supplied onto the surface of the inspection target from the water supply devices in the advancing direction and form streamlines for supplying the water between inspection sensors configured to inspect the inspection target for defects and the surface of the inspection target simultaneously with the movement of the body.