To suppress seizure of a sliding part caused at the time of an operation by priming, deterioration of a material, and generation of frictional noise and the like by forming a groove from a center to an outer periphery at an upper position from a center of a pump chamber on a pump chamber side wall so as to shut out flow of circulating water in water passage parts of an outer periphery and a back surface of the pump chamber and an impeller.

A groove 17 is formed on a pump chamber 7 side wall of a casing 3 from a center to an outer periphery at an upper position from a center of the chamber 7 so as to shut out flow of circulating water in water passage parts of an outer periphery and a back surface of the pump chamber 7 and an impeller. Even at the time of an operation by priming into a pump, flow of the circulating water in an outer peripheral part of the impeller 6 is changed, a part of circulating water is supplied to a sliding part of a rotation side mechanical seal and a fixing side mechanical seal by passing the groove 17. It is thus possible to suppress seizure of the sliding part, deterioration of a material, and generation of frictional noise and the like.