A turbomachine, comprising: a rotational shaft; a pair of thrust collars (20A, 20B) disposed around the rotational shaft; a thrust bearing disposed around the rotational shaft at an axial directional position between the pair of thrust collars; and an annular channel disposed around the rotational shaft in an annular shape, for draining lubricant oil after lubricating a sliding portion between the thrust bearing and the thrust collars, wherein the annular channel includes: an oil-drain port portion forming a lower region of the annular channel, for discharging the lubricant oil inside the annular channel downward; and an oil-guide channel portion disposed above the oil-drain port portion so as to form an upper region of the annular channel and configured to guide the lubricant oil from the sliding portion in a circumferential direction of the rotational shaft to the oil-drain port portion, and wherein the oil-guide channel portion forming the upper region of the annular channel has an asymmetric shape with respect to a rotational direction of the rotational shaft, the oil-guide channel portion having a minimum flow-path cross sectional area at a first position on an upstream side in the rotational direction, and a maximum flow-path cross sectional area at a second position on a downstream side of the first position, characterized in that the oil-guide channel portion is configured such that the flow-path cross sectional area increases monotonically and continuously from the first position toward the second position of the oil-guide channel portion along the rotational direction. a rotational shaft; a pair of thrust collars (20A, 20B) disposed around the rotational shaft; a thrust bearing disposed around the rotational shaft at an axial directional position between the pair of thrust collars; and an annular channel disposed around the rotational shaft in an annular shape, for draining lubricant oil after lubricating a sliding portion between the thrust bearing and the thrust collars, wherein the annular channel includes: an oil-drain port portion forming a lower region of the annular channel, for discharging the lubricant oil inside the annular channel downward; and an oil-guide channel portion disposed above the oil-drain port portion so as to form an upper region of the annular channel and configured to guide the lubricant oil from the sliding portion in a circumferential direction of the rotational shaft to the oil-drain port portion, and an oil-drain port portion forming a lower region of the annular channel, for discharging the lubricant oil inside the annular channel downward; and an oil-guide channel portion disposed above the oil-drain port portion so as to form an upper region of the annular channel and configured to guide the lubricant oil from the sliding portion in a circumferential direction of the rotational shaft to the oil-drain port portion, and wherein the oil-guide channel portion forming the upper region of the annular channel has an asymmetric shape with respect to a rotational direction of the rotational shaft, the oil-guide channel portion having a minimum flow-path cross sectional area at a first position on an upstream side in the rotational direction, and a maximum flow-path cross sectional area at a second position on a downstream side of the first position, characterized in that the oil-guide channel portion is configured such that the flow-path cross sectional area increases monotonically and continuously from the first position toward the second position of the oil-guide channel portion along the rotational direction.