An outdoor unit, comprising: a base metal plate having a base water-discharge hole; a plurality of outer shell panels (6a-6e) surrounding the base metal plate; a first partition plate mounted to the base metal plate so as to extend upward; a second partition plate mounted to one surface of the first partition plate to be parallel to the base metal plate; a third partition plate mounted to the base metal plate to extend upward, and has an upper end portion continuous with the second partition plate; an air-sending device chamber formed in a casing formed of the base metal plate and the plurality of outer shell panels (6a-6e) and partitioned by the first partition plate, and being a space opposite to the second partition plate across the first partition plate; an electric component chamber formed in the casing and partitioned by the first partition plate and the third partition plate, and being a space defined by a surface of the first partition plate to which the second partition plate is mounted and an upper surface of the second partition plate; and a machine chamber formed in the casing and partitioned by the first partition plate and the third partition plate, and being a space defined by the surface of the first partition plate to which the second partition plate is mounted and a lower surface of the second partition plate, the electric component chamber including the base water-discharge hole, a first airflow hole formed in at least one of the plurality of outer shell panels (6a-6e), and a second airflow hole formed in the first partition plate, communicating with the first airflow hole, and defining an air passage extending upward along the third partition plate between the first airflow hole and the second airflow hole, the machine chamber having a sealed structure without an air intake hole, characterized in that the second partition plate is continuous with an upper end portion of the third partition plate, and has a cutout which defines the air passage. a base metal plate having a base water-discharge hole; a plurality of outer shell panels (6a-6e) surrounding the base metal plate; a first partition plate mounted to the base metal plate so as to extend upward; a second partition plate mounted to one surface of the first partition plate to be parallel to the base metal plate; a third partition plate mounted to the base metal plate to extend upward, and has an upper end portion continuous with the second partition plate; an air-sending device chamber formed in a casing formed of the base metal plate and the plurality of outer shell panels (6a-6e) and partitioned by the first partition plate, and being a space opposite to the second partition plate across the first partition plate; an electric component chamber formed in the casing and partitioned by the first partition plate and the third partition plate, and being a space defined by a surface of the first partition plate to which the second partition plate is mounted and an upper surface of the second partition plate; and a machine chamber formed in the casing and partitioned by the first partition plate and the third partition plate, and being a space defined by the surface of the first partition plate to which the second partition plate is mounted and a lower surface of the second partition plate, the electric component chamber including the base water-discharge hole, a first airflow hole formed in at least one of the plurality of outer shell panels (6a-6e), and a second airflow hole formed in the first partition plate, communicating with the first airflow hole, and defining an air passage extending upward along the third partition plate between the first airflow hole and the second airflow hole, the base water-discharge hole, a first airflow hole formed in at least one of the plurality of outer shell panels (6a-6e), and a second airflow hole formed in the first partition plate, communicating with the first airflow hole, and defining an air passage extending upward along the third partition plate between the first airflow hole and the second airflow hole, the machine chamber having a sealed structure without an air intake hole, characterized in that the second partition plate is continuous with an upper end portion of the third partition plate, and has a cutout which defines the air passage.