To enhance discharge performance in low-speed rotation of a water pump and to reduce the discharge performance in high-speed rotation, and to prevent a reduction in the discharge performance by other members.

An impeller in the water pump includes a storage case 1 with a plurality of long grooves 12a formed radially on a bottom disk 12, an impeller base 2 with blade sliding long holes 23 formed radially, which is fixed on the storage case 1, blade bodies 3 each with a slide shaft 33 arranged in the lower surface of a slide plate 32 in which a unit blade plate 31 is stood, and weights 4 each with notch long grooves 41 formed on both sides and a slide square shaft 43 sliding in the long groove 12a, which is arranged in its lower end. A plurality of the weights 4 stored in the storage case 1 resiliently constitute centrifugal force by compression springs 5. The slide plates 32 of the blade bodies 3 are arranged slidably correspondingly to the respective blade sliding long holes 23. The slide shafts 33 are arranged slidably in the notch long grooves 41 of the weights 4, The slide shafts 33 are configured to be positioned outside the impeller base 2 in a low rotation region of an engine by the resilient force of the compression springs 5.