To provide a horizontal shaft pump wherein its installation work is simplified by shortening a dimension required for the installation in the direction of a pump shaft 20, reducing an installation space, and allowing centering adjusting work of the pump shaft 20 and a speed reducer 40 in a manufacturing factory.

The speed reducer 40 is integrally attached to an outer wall of a suction elbow 12 where the pump shaft 20 gets through and projects in the horizontal direction. The pump shaft 20 projecting from the suction elbow 12 is inserted into a casing 42 of the speed reducer 40. The pump shaft 20 is journaled to a bearing in the casing 42, and an output stage of a gear train of the speed reducer 40 is connected.