The present invention relates to water pump including a main body portion including a housing having an inlet and an outlet for coolant inflow and discharge, an impeller accommodated in the housing for rotation-based coolant inflow and discharge, a rotary shaft coupled with the impeller and rotating the impeller by receiving an external driving force, and coolant flow rate control means disposed on the upper side of the impeller and operated for selective opening and closing of the outlet; a cylinder portion formed on the upper side of the coolant flow rate control means and having an inner hydraulic space accommodating a coolant applying a pressure to the coolant flow rate control means; a piston positioned in the hydraulic space and applying a pressure to the coolant in the hydraulic space; a driving unit coupled with a side surface of the cylinder portion and moving the piston forward and backward; and a lock prevention member formed of an elastic material and positioned on an inside surface of the cylinder portion facing the front or rear surface of the piston. The water pump according to the present invention is provided with the lock prevention member, and thus a situation can be forestalled in which the piston is immovable in contact with the inner wall of the cylinder portion.
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