A water saving aerator adapted to be attached to a water tap for reducing the amount of water flow comprising: a water saving disc covering a water tap outflow opening, the water saving disc having at least one through-hole whose diameter is smaller than that of the water tap for obtaining a water saving effect by limiting the total water outflow cross-sectional area to be less than the water tap cross-sectional area; a diffuser object for diffusing the water coming from the water saving disc in a peripheral direction, wherein the diffuser object is extended from and suspended from a lower surface of the water saving disc, the diffuser object comprising a diffuser plate having a skew whose height falls in the peripheral direction, and a lower object extended vertically downward from an outer edge of the diffuser plate, the diffuser plate being located in a place facing the through-hole of the water saving disc, and diffusing the water flow coming from the through-hole in the peripheral direction; a first cylinder for covering the diffuser object from outside, receiving the water flow diffused by the diffuser object by an inner wall, and leading the received water flow along the inner wall to a water pass, wherein the lower object is facing to the inner wall of the first cylinder and extends along the water flow, and the water pass is formed between an outer wall of the lower object of the diffuser object and the inner wall of the first cylinder, wherein the water pass is filled with water, when the water flow flows; an air cavity leading to outer air is formed in the first cylinder under the water saving disc and above the diffuser object, the water flow is injected to the air in the air cavity from the through-hole of the water saving disc and the water flow is led to the diffuser object through air in the air cavity, a second cylinder covering the first cylinder from the outside and providing a gap between the inner wall of the second cylinder and the outer wall of the first cylinder, an air intake hole being formed in the upper portion of the wall of the first cylinder higher than the diffuser object and the water pass, wherein a ventilation pass for ventilating the outer air is formed by the gap between the inner wall of the second cylinder and the outer wall of the first cylinder to the air cavity via the air intake hole, and the air flow entering into the air cavity through the ventilation pass via the air intake hole is generated by utilizing the decrease of the air pressure by the water flow and the air flow from the air cavity to the water pass. a water saving disc covering a water tap outflow opening, the water saving disc having at least one through-hole whose diameter is smaller than that of the water tap for obtaining a water saving effect by limiting the total water outflow cross-sectional area to be less than the water tap cross-sectional area; a diffuser object for diffusing the water coming from the water saving disc in a peripheral direction, wherein the diffuser object is extended from and suspended from a lower surface of the water saving disc, the diffuser object comprising a diffuser plate having a skew whose height falls in the peripheral direction, and a lower object extended vertically downward from an outer edge of the diffuser plate, the diffuser plate being located in a place facing the through-hole of the water saving disc, and diffusing the water flow coming from the through-hole in the peripheral direction; a first cylinder for covering the diffuser object from outside, receiving the water flow diffused by the diffuser object by an inner wall, and leading the received water flow along the inner wall to a water pass, wherein the lower object is facing to the inner wall of the first cylinder and extends along the water flow, and the water pass is formed between an outer wall of the lower object of the diffuser object and the inner wall of the first cylinder, wherein the water pass is filled with water, when the water flow flows; an air cavity leading to outer air is formed in the first cylinder under the water saving disc and above the diffuser object, the water flow is injected to the air in the air cavity from the through-hole of the water saving disc and the water flow is led to the diffuser object through air in the air cavity, a second cylinder covering the first cylinder from the outside and providing a gap between the inner wall of the second cylinder and the outer wall of the first cylinder, an air intake hole being formed in the upper portion of the wall of the first cylinder higher than the diffuser object and the water pass, wherein a ventilation pass for ventilating the outer air is formed by the gap between the inner wall of the second cylinder and the outer wall of the first cylinder to the air cavity via the air intake hole, and the air flow entering into the air cavity through the ventilation pass via the air intake hole is generated by utilizing the decrease of the air pressure by the water flow and the air flow from the air cavity to the water pass.