PURPOSE: To simplify construction by providing a diffuser casing which forms a delivery passage and an inner casing which forms a suction passage, in a cylindrical casing of a vertical shaft double suction pump.

CONSTITUTION: The cylindrical part 44b of an inner casing 44 can be provided in an integrated form with a protecting tube 48 and a pump cover 41, and an outer casing 50 which forms a cylindrical pump casing part, forms a stepped part 51 on the upper part of the cylindrical part 44b of the inner casing 44, enabling the inner casing 44 to be easily drawn out. The outer periphery of a diffuser casing 43 constitutes a part of cylindrical pump casing, and is fixed to the outer casing 50 with a bolt. Thereby, the construction is simplified making the device compact.