A foam discharge device comprising: a pump having a stem downwardly movably erected at a mouth section of a container main body in which a liquid is accommodated in an upwardly biased state, and a pressing head disposed at an upper end section of the stem and in which a nozzle hole is formed; and a mounting cap configured to mount the pump on the mouth section of the container main body, and configured to foam and discharge the liquid in the container main body from the nozzle hole, wherein the pump is provided with: a piston for a liquid linked to the stem; a cylinder for a liquid in which the piston for a liquid is vertically slidably accommodated; a piston for air linked to the stem; a cylinder for air in which the piston for air is vertically slidably accommodated; and an air-liquid mixing unit configured to mix a liquid from the cylinder for a liquid and air from the cylinder for air to foam the liquid, the cylinder for air is disposed inside the mounting cap and accommodated in the container main body, characterized in that a head-section-side cylinder for air into which a piston tube section erected at a ceiling wall section of the mounting cap is vertically slidably fitted extends from the pressing head, wherein the pressing head is provided with a mounting tube section extending downward into which the stem is fitted and an outer tube section extending downward configured to enclose the mounting tube section from the outside in a radial direction of the mounting tube section and divide the head-side cylinder for air, a communication path configured to bring the inside of the head-section-side cylinder for air and the inside of the cylinder for air in communication with each other is formed at the pump, and wherein a part of the communication path is constituted by a longitudinal communication groove formed at an inner circumferential surface of a passage tube section disposed between the mounting tube section and the outer tube section, or by a communication gap formed between the mounting tube section and the piston tube section, and opening in the upward and downward directions. a pump having a stem downwardly movably erected at a mouth section of a container main body in which a liquid is accommodated in an upwardly biased state, and a pressing head disposed at an upper end section of the stem and in which a nozzle hole is formed; and a mounting cap configured to mount the pump on the mouth section of the container main body, and configured to foam and discharge the liquid in the container main body from the nozzle hole, wherein the pump is provided with: a piston for a liquid linked to the stem; a cylinder for a liquid in which the piston for a liquid is vertically slidably accommodated; a piston for air linked to the stem; a cylinder for air in which the piston for air is vertically slidably accommodated; and an air-liquid mixing unit configured to mix a liquid from the cylinder for a liquid and air from the cylinder for air to foam the liquid, the cylinder for air is disposed inside the mounting cap and accommodated in the container main body, characterized in that a head-section-side cylinder for air into which a piston tube section erected at a ceiling wall section of the mounting cap is vertically slidably fitted extends from the pressing head, wherein the pressing head is provided with a mounting tube section extending downward into which the stem is fitted and an outer tube section extending downward configured to enclose the mounting tube section from the outside in a radial direction of the mounting tube section and divide the head-side cylinder for air, a communication path configured to bring the inside of the head-section-side cylinder for air and the inside of the cylinder for air in communication with each other is formed at the pump, and wherein a part of the communication path is constituted by a longitudinal communication groove formed at an inner circumferential surface of a passage tube section disposed between the mounting tube section and the outer tube section, or by a communication gap formed between the mounting tube section and the piston tube section, and opening in the upward and downward directions. a piston for a liquid linked to the stem; a cylinder for a liquid in which the piston for a liquid is vertically slidably accommodated; a piston for air linked to the stem; a cylinder for air in which the piston for air is vertically slidably accommodated; and an air-liquid mixing unit configured to mix a liquid from the cylinder for a liquid and air from the cylinder for air to foam the liquid, the cylinder for air is disposed inside the mounting cap and accommodated in the container main body, characterized in that a head-section-side cylinder for air into which a piston tube section erected at a ceiling wall section of the mounting cap is vertically slidably fitted extends from the pressing head, wherein the pressing head is provided with a mounting tube section extending downward into which the stem is fitted and an outer tube section extending downward configured to enclose the mounting tube section from the outside in a radial direction of the mounting tube section and divide the head-side cylinder for air, a communication path configured to bring the inside of the head-section-side cylinder for air and the inside of the cylinder for air in communication with each other is formed at the pump, and wherein