Cargo bed structure of truck

A cargo bed structure of a truck has concave portions (6) formed within an inside surface of each right and left side wall (3) for detachably mounting scantlings on a lower surface of the concave portion (6), respectively. The bed structure includes a rotational member (8), an end side of which is supported rotatably by a surface of the side wall formed over the concave portion. The rotational member (8) regulates a movement of the scantlings in a vertical direction with the other end of the rotational member (8) when the rotational member (8) rotates in one direction, and deregulates the movement of the scantlings when the rotational member (8) rotates in the other direction.