An outdoor unit for an air-conditioning apparatus, comprising: a heat exchanger, which is provided in a main body of the outdoor unit and is configured to exchange heat between refrigerant flowing through a heat transfer tube and air to be taken in; a bottom plate forming a bottom surface of an outer shell of the main body; and a separation member, which is arranged on the bottom plate, receives the heat exchanger placed on the separation member, and is configured to separate the bottom plate and the heat exchanger from each other, the separation member being formed of a metal member, which is electrically less noble than a member forming the heat exchanger, or a resin member, characterized in that the bottom plate includes a drainage passage, which is formed at a position corresponding to an arrangement position of the heat exchanger, is formed so as to protrude downward in a vertical direction of the bottom plate, and is configured to drain water including drain water generated in the heat exchanger, and one or a plurality of drainage holes, which are each formed so as to protrude downward in the vertical direction from the drainage passage and are each configured to discharge the water flowing through the drainage passage to an outside, the drainage passage comprising a drainage surface inclined downward in the vertical direction toward one of the one or the plurality of drain holes, the drainage passage having a width being a dimension in a direction corresponding to a direction of flow of the air passing through the heat exchanger, which is larger than a width of the heat exchanger, the width being a dimension in a direction corresponding to the direction of flow of the air, the separation member being provided in the drainage passage and having a shape of preventing closing of the drainage passage, wherein a height from a reference position of the drainage passage to a surface on which the heat exchanger is placed is set larger than a height from the reference position to an upper surface of the drainage passage. a heat exchanger, which is provided in a main body of the outdoor unit and is configured to exchange heat between refrigerant flowing through a heat transfer tube and air to be taken in; a bottom plate forming a bottom surface of an outer shell of the main body; and a separation member, which is arranged on the bottom plate, receives the heat exchanger placed on the separation member, and is configured to separate the bottom plate and the heat exchanger from each other, the separation member being formed of a metal member, which is electrically less noble than a member forming the heat exchanger, or a resin member, characterized in that the bottom plate includes a drainage passage, which is formed at a position corresponding to an arrangement position of the heat exchanger, is formed so as to protrude downward in a vertical direction of the bottom plate, and is configured to drain water including drain water generated in the heat exchanger, and one or a plurality of drainage holes, which are each formed so as to protrude downward in the vertical direction from the drainage passage and are each configured to discharge the water flowing through the drainage passage to an outside, the drainage passage comprising a drainage surface inclined downward in the vertical direction toward one of the one or the plurality of drain holes, the drainage passage having a width being a dimension in a direction corresponding to a direction of flow of the air passing through the heat exchanger, which is larger than a width of the heat exchanger, the width being a dimension in a direction corresponding to the direction of flow of the air, the separation member being provided in the drainage passage and having a shape of preventing closing of the drainage passage, wherein a height from a reference position of the drainage passage to a surface on which the heat exchanger is placed is set larger than a height from the reference position to an upper surface of the drainage passage.