An air trap chamber comprising: a chamber body that has a substantially cylindrical shape, has an inlet pipe at one end with respect to a direction of the central axis and an outlet at another end, and in which a liquid flows down from the inlet pipe to the outlet downward, wherein the inlet pipe is extended into the chamber body and an inlet, an end opening of the inlet pipe, is provided at an inner circumferential surface of the chamber body and faced in a circumferential direction, a filter covering the outlet is provided in the chamber body, the filter includes: a cylindrical section that surrounds the outlet and extends in the direction of the central axis of the chamber body, a ceiling section that covers an upper end of the cylindrical section, the upper end opposite to a lower end of the chamber body, the lower end being adjacent to the outlet, the cylindrical section of the filter has openings in multiple stages aligned in the direction of the central axis, and characterized in that a circumferential width of the opening in an upper stage adjacent to the ceiling section of the filter is made larger than a circumferential width of the opening in a lower stage adjacent to the outlet, and in that the filter is provided with multiple ribs (44,45) that extend in the direction of the central axis and are aligned along the circumferential direction, and the multiple ribs (44,45) include: a first rib that extends from the lower end of the filter to the ceiling section of the filter; and a second rib that extends from the lower end of the filter and terminates before reaching the ceiling section of the filter. a chamber body that has a substantially cylindrical shape, has an inlet pipe at one end with respect to a direction of the central axis and an outlet at another end, and in which a liquid flows down from the inlet pipe to the outlet downward, wherein the inlet pipe is extended into the chamber body and an inlet, an end opening of the inlet pipe, is provided at an inner circumferential surface of the chamber body and faced in a circumferential direction, a filter covering the outlet is provided in the chamber body, the filter includes: a cylindrical section that surrounds the outlet and extends in the direction of the central axis of the chamber body, a ceiling section that covers an upper end of the cylindrical section, the upper end opposite to a lower end of the chamber body, the lower end being adjacent to the outlet, a cylindrical section that surrounds the outlet and extends in the direction of the central axis of the chamber body, a ceiling section that covers an upper end of the cylindrical section, the upper end opposite to a lower end of the chamber body, the lower end being adjacent to the outlet, the cylindrical section of the filter has openings in multiple stages aligned in the direction of the central axis, and characterized in that a circumferential width of the opening in an upper stage adjacent to the ceiling section of the filter is made larger than a circumferential width of the opening in a lower stage adjacent to the outlet, and in that the filter is provided with multiple ribs (44,45) that extend in the direction of the central axis and are aligned along the circumferential direction, and the multiple ribs (44,45) include: a first rib that extends from the lower end of the filter to the ceiling section of the filter; and a second rib that extends from the lower end of the filter and terminates before reaching the ceiling section of the filter. a first rib that extends from the lower end of the filter to the ceiling section of the filter; and a second rib that extends from the lower end of the filter and terminates before reaching the ceiling section of the filter.