Impeller for water pump

A present invention's object is to maintain a discharge performance by making a discharge flow rate constant in a low rotation region of an engine rotation speed and to reduce the discharge flow rate in a high rotation region. The present invention consists of a housing case, an impeller base provided with plural holes and plural elliptical elongated holes, a vane body provided with a rotary shaft on an inner peripheral side and a rocking shaft on an outer peripheral side, a plate cam in which an elongated groove is formed on an outer peripheral side of a disc portion and a single torsion spring. The spring and the plate cam are housed in the housing case and the impeller base. The rotary shaft is inserted rotatably into the holes, and the rocking shaft is movably inserted into the elliptical elongated holes and the elongated groove.