To provide a horizontal shaft pump in which the height of a rise part 42a formed on a suction pipe 42 is reduced to prevent water from flowing into a vacuum pump 52.

The rise part 42a is formed on the suction pipe 42. A control means 64 sets a pressure in the suction pipe 42 to a first minimum pressure P1 kPa when the water level of a suction water tank 18 is within the range of 1/2 of the difference between the minimum and maximum levels according to a signal from a water level gauge 66 disposed in the suction water tank 18, and to a second minimum pressure P2 kPa within the range of the difference between the middle level and the maximum level. The height h1 m of the top point of the rise part 42a from a suction hole 38, the height h2 m of the suction hole 38 from the maximum water level of the suction water tank 18, and the height h3 m of the maximum water level from the minimum water level of the suction water tank 18 are set to be h1 + h2 + h3 x 1/2 10 - P1/9.8 h2 + h3 and h1 + h2 10 - P2/9.8 h2 + h3 x 1/2.