PURPOSE: To simplify locking operation by a method wherein in the locking device of dump door on a truck etc., a forked connecting link is fixed to the bottom of a handle and a locking lever is connected with the connecting hole of a fixing column by the horizontal revolution of the handle.

CONSTITUTION: At the upper end of a hinge part 20 is fixed a horizontal connecting link 22 which is provided in a forked shape and a connecting shaft 17 is interlocked in a groove 23. When a handle part 21 is kept in parallel with a side dump door 6, the connecting link 22 makes a locking lever 14 project from the side dump door 6, the locking lever 14 is inserted into a connecting hole 28 and the side dump door 6 is locked in the fixed column 3. When the handle part is revolved horizontally against the side of a container, the connecting link 22 turns toward the center of the side dump door 6, the locking lever 14 is sunk in the upper convex portion, leaves the fixed column 3 and the side dump door 6 can easily be released from locking.