A bubble separator (20, 120, 220, 320, 420, 520, 620) used for a fluid circuit for an automobile and separating bubbles in liquid, comprising: a swirl flow formation part (22, 122, 222, 322, 422, 522, 622) that includes a columnar-shaped internal space; a flow inlet (24, 124, 224, 324, 424, 524, 624) disposed at one end of the swirl flow formation part (22, 122, 222, 322, 422, 522, 622), the flow inlet (24, 124, 224, 324, 424, 524, 624) opening so as to cause the liquid to flow in in a tangential direction of an inner peripheral surface of the swirl flow formation part (22, 122, 222, 322, 422, 522, 622) to form a swirl flow on the inner peripheral surface; a flow outlet (26, 126, 226, 326, 426, 526, 626) disposed at another end of the swirl flow formation part (22, 122, 222, 322, 422, 522, 622), the flow outlet 26,126,226,326,426,526,626) opening so as to cause the liquid to flow out in the tangential direction from the inner peripheral surface; and gas discharge ports (30, 130, 230, 330, 430, 530, 630, 32, 132, 232, 332, 432, 532, 632) that discharge gas separated from the liquid at the swirl flow formation part (22, 122, 222, 322, 422, 522, 622) to an outside of the swirl flow formation part (22, 122, 222, 322, 422, 522, 622), wherein the gas discharge ports (30, 130, 230, 330, 430, 530, 630, 32, 132, 232, 332, 432, 532, 632) are respectively provided at both ends of the swirl flow formation part (22, 122, 222, 322, 422, 522, 622), wherein the bubble separator (20, 120, 220, 320, 420, 520, 620) comprises a tubular gas column promotion part (28, 428) that extends coaxially with the swirl flow formation part (22, 122, 222, 322, 422, 522, 622) in the swirl flow formation part (22, 122, 222, 322, 422, 522, 622) characterized in that, when an inner diameter of the flow inlet (24, 124, 224, 324, 424, 524, 624) is d 1, an inner diameter D of the swirl flow formation part (22, 122, 222, 322, 422, 522, 622) ranges from 1.5d 1 to 3d 1. a swirl flow formation part (22, 122, 222, 322, 422, 522, 622) that includes a columnar-shaped internal space; a flow inlet (24, 124, 224, 324, 424, 524, 624) disposed at one end of the swirl flow formation part (22, 122, 222, 322, 422, 522, 622), the flow inlet (24, 124, 224, 324, 424, 524, 624) opening so as to cause the liquid to flow in in a tangential direction of an inner peripheral surface of the swirl flow formation part (22, 122, 222, 322, 422, 522, 622) to form a swirl flow on the inner peripheral surface; a flow outlet (26, 126, 226, 326, 426, 526, 626) disposed at another end of the swirl flow formation part (22, 122, 222, 322, 422, 522, 622), the flow outlet 26,126,226,326,426,526,626) opening so as to cause the liquid to flow out in the tangential direction from the inner peripheral surface; and gas discharge ports (30, 130, 230, 330, 430, 530, 630, 32, 132, 232, 332, 432, 532, 632) that discharge gas separated from the liquid at the swirl flow formation part (22, 122, 222, 322, 422, 522, 622) to an outside of the swirl flow formation part (22, 122, 222, 322, 422, 522, 622), wherein the gas discharge ports (30, 130, 230, 330, 430, 530, 630, 32, 132, 232, 332, 432, 532, 632) are respectively provided at both ends of the swirl flow formation part (22, 122, 222, 322, 422, 522, 622), wherein the bubble separator (20, 120, 220, 320, 420, 520, 620) comprises a tubular gas column promotion part (28, 428) that extends coaxially with the swirl flow formation part (22, 122, 222, 322, 422, 522, 622) in the swirl flow formation part (22, 122, 222, 322, 422, 522, 622) characterized in that, when an inner diameter of the flow inlet (24, 124, 224, 324, 424, 524, 624) is d 1, an inner diameter D of the swirl flow formation part (22, 122, 222, 322, 422, 522, 622) ranges from 1.5d 1 to 3d 1.