PURPOSE: To prevent a shock due to vertical motion of the bottom of a ship by disposing horizontal rails on a horizontal digging portion frame supporting front and rear sprockets and freely engaging and disengaging rollers provided on the upper surface of a bucket with and from the rails by the vertical motion of the frame in the above unloader for a ship.

CONSTITUTION: At the time of ordinary side digging, inclined faces 2A, 2B of rollers 2 are guided by rails 4 horizontal in the longitudinal direction to rotate a bucket 1. Thus, the lateral force and vertical force applied to the bucket 1 can be received by the rails 4. Accordingly, a bucket chain 5 may have only rigidity for tracting the bucket 1. In case of catenary bottom catenary, a frame 10 is lifted up to separate the rollers 2 from guide grooves 4A, 4B of the rails 4. Accordingly, an unloader can move corresponding to the up-and- down motion of the bottom of a ship so as to prevent a shock.