In the case of small vessels, the change of breadth of long hatches has been observed. In this paper the mechanism of this change is studied theoretically, and is confirmed by experiments using M. S. TETSURYU-MARU. The change is determined as the sum of horizontal displacements of the deck on each side of hatchway, and is partly due to the horizontal action from web frame to the deck and partly due to the shearing force acting at the ship sides. The deck is fixed elastically at the hatch ends, and the change of hatch breadth is almost equal to twice as large as that calculated assuming the deck is fixed at the both ends.