A foam dispenser (100, 200, 300), comprising: a placing member that includes a placing tubular portion adapted to be fitted to an outer surface of a neck of a container and an inwardly directed flange-shaped top wall portion provided in an upper portion of the placing tubular portion; a cylinder that includes a lower portion and an upper portion and that is suspended from a lower surface of the inwardly directed flange-shaped top wall portion into the container, the lower portion of the cylinder being formed as a small-diameter liquid cylinder portion and the upper portion of the cylinder being formed as an air cylinder portion, the air cylinder portion having a larger diameter than the small-diameter liquid cylinder portion; and an actuator that includes a liquid piston adapted to slide in the liquid cylinder portion, a stem standing from the cylinder, a ring-shaped partition wall surrounding a middle portion of the stem in a vertical direction thereof, an air piston provided on an outer circumferential portion of the partition wall, the outer circumferential portion constitutes an outer circumferential wall that extends upward from a middle portion of the air piston in the vertical direction thereof, and a depression head placed over the stem and including a nozzle, wherein the liquid piston is slidably fitted in the liquid cylinder portion, and the air piston is slidably fitted in the air cylinder portion, and upward and downward displacement of the actuator causes a content liquid within the liquid cylinder portion and air within the air cylinder portion to be mixed and foamed and then dispensed from the nozzle, an inner circumferential wall is suspended from a position inward of a middle portion of the partition wall in a radial direction thereof, and an air suction valve including a valve tube (V3a) and an elastic valve plate (V3b) is formed between the inner circumferential wall and the outer circumferential wall, the valve tube being fitted to the inner circumferential wall, the elastic valve plate protruding from an outer circumferential surface of the valve tube, the partition wall is provided with an ambient air introducing hole, and from at least a portion of a hole periphery of the ambient air introducing hole, a water entering prevention side wall portion stands for preventing water from entering the ambient air introducing hole, and the inwardly directed flange-shaped top wall portion includes a flange hole from which a suspended tubular portion is suspended, and the ambient air introducing hole is provided outward of the suspended tubular portion in the radial direction of the partition wall. a placing member that includes a placing tubular portion adapted to be fitted to an outer surface of a neck of a container and an inwardly directed flange-shaped top wall portion provided in an upper portion of the placing tubular portion; a cylinder that includes a lower portion and an upper portion and that is suspended from a lower surface of the inwardly directed flange-shaped top wall portion into the container, the lower portion of the cylinder being formed as a small-diameter liquid cylinder portion and the upper portion of the cylinder being formed as an air cylinder portion, the air cylinder portion having a larger diameter than the small-diameter liquid cylinder portion; and an actuator that includes a liquid piston adapted to slide in the liquid cylinder portion, a stem standing from the cylinder, a ring-shaped partition wall surrounding a middle portion of the stem in a vertical direction thereof, an air piston provided on an outer circumferential portion of the partition wall, the outer circumferential portion constitutes an outer circumferential wall that extends upward from a middle portion of the air piston in the vertical direction thereof, and a depression head placed over the stem and including a nozzle, wherein the liquid piston is slidably fitted in the liquid cylinder portion, and the air piston is slidably fitted in the air cylinder portion, and upward and downward displacement of the actuator causes a content liquid within the liquid cylinder portion and air within the air cylinder portion to be mixed and foamed and then dispensed from the nozzle, an inner circumferential wall is suspended from a position inward of a middle portion of the partition wall in a radial direction thereof, and an air suction valve including a valve tube (V3a) and an elastic valve plate (V3b) is formed between the inner circumferential wall and the outer circumferential wall, the valve tube being fitted to the inner circumferential wall, the elastic valve plate protruding from an outer circumferential surface of the valve tube, the partition wall is provided with an ambient air introducing hole, and from at least a portion of a hole periphery of the ambient air introducing hole, a water entering prevention side wall portion stands for preventing water from entering the ambient air introducing hole, and the inwardly directed flange-shaped top wall portion includes a flange hole from which a suspended tubular portion is suspended, and the ambient air introducing hole is provided outward of the suspended tubular portion in the radial direction of the partition wall.