An indoor unit of an air-conditioning apparatus, the indoor unit comprising: a base forming a rear surface and a part of a top surface of a main body; and a design panel removably attached to at least one of a left side and a right side of the base to form a part of the top surface, a part of a lower surface, and a side surface of the main body, wherein the base includes a first corner portion connecting a top surface and a side surface of the base, and a step portion formed on the top surface of the base, wherein the design panel includes a second corner portion connecting a top surface and a side surface of the design panel, and a cutout portion formed in the top surface of the design panel, the indoor unit characterized in that when the design panel is attached to the base the step portion has a top surface portion (1a-2) exposed through the cutout portion, with the top surface portion (1a-2) and the design panel forming the top surface of the main body, and the design panel forms the side surface of the main body, with the second corner portion covering the first corner portion, and the step portion includes a wall portion (1a-1) projecting outward of an end portion of the first corner portion, and the top surface portion (1a-2) continuously extending from the wall portion (1a-1) in a width direction of the base, and when the design panel is attached to the base, the top surface portion (1a-2) and the design panel form a flat top surface of the main body. a base forming a rear surface and a part of a top surface of a main body; and a design panel removably attached to at least one of a left side and a right side of the base to form a part of the top surface, a part of a lower surface, and a side surface of the main body, wherein the base includes a first corner portion connecting a top surface and a side surface of the base, and a step portion formed on the top surface of the base, a first corner portion connecting a top surface and a side surface of the base, and a step portion formed on the top surface of the base, wherein the design panel includes a second corner portion connecting a top surface and a side surface of the design panel, and a cutout portion formed in the top surface of the design panel, the indoor unit characterized in that a second corner portion connecting a top surface and a side surface of the design panel, and a cutout portion formed in the top surface of the design panel, the indoor unit characterized in that when the design panel is attached to the base the step portion has a top surface portion (1a-2) exposed through the cutout portion, with the top surface portion (1a-2) and the design panel forming the top surface of the main body, and the design panel forms the side surface of the main body, with the second corner portion covering the first corner portion, and the step portion includes a wall portion (1a-1) projecting outward of an end portion of the first corner portion, and the top surface portion (1a-2) continuously extending from the wall portion (1a-1) in a width direction of the base, and the step portion has a top surface portion (1a-2) exposed through the cutout portion, with the top surface portion (1a-2) and the design panel forming the top surface of the main body, and the design panel forms the side surface of the main body, with the second corner portion covering the first corner portion, and the step portion includes a wall portion (1a-1) projecting outward of an end portion of the first corner portion, and the top surface portion (1a-2) continuously extending from the wall portion (1a-1) in a width direction of the base, and when the design panel is attached to the base, the top surface portion (1a-2) and the design panel form a flat top surface of the main body.