An outdoor unit for an air-conditioning apparatus, the outdoor unit comprising: a casing that has a front panel in which an opening hole is opened; a fan provided to the casing and configured to blow out air to an outside through the opening hole (la); a roof part installed at a front of the front panel and above the opening hole and extending in a width direction of the casing; and an outlet grille fixed to the casing and covering the opening hole, the roof part having a main roof portion that extends further forward than the front panel, and a sub-roof portion that extends downward and forward from the main roof portion and has a recession formed between a bottom face of the main roof portion and the sub-roof portion, the outlet grille being located such that an upper end portion of the outlet grille is in contact with the recession, the front panel and the upper end portion of the outlet grille holding the roof part such that the front panel and the upper end portion of the outlet grille sandwich the roof part, the outdoor unit being characterised by the casing having a top plate provided on top of the front panel, the top plate having a top-plate extension portion that extends further forward than a tail end of the main roof portion and covering a portion of the roof part that is in the vicinity of the front panel, the top-plate extension portion having a flange portion that protrudes downward, a top face of the main roof portion and a lower edge (3 1a) of the flange portion being in contact with each other in a state in which the outlet grille is fixed to the casing. a casing that has a front panel in which an opening hole is opened; a fan provided to the casing and configured to blow out air to an outside through the opening hole (la); a roof part installed at a front of the front panel and above the opening hole and extending in a width direction of the casing; and an outlet grille fixed to the casing and covering the opening hole, the roof part having a main roof portion that extends further forward than the front panel, and a sub-roof portion that extends downward and forward from the main roof portion and has a recession formed between a bottom face of the main roof portion and the sub-roof portion, a main roof portion that extends further forward than the front panel, and a sub-roof portion that extends downward and forward from the main roof portion and has a recession formed between a bottom face of the main roof portion and the sub-roof portion, the outlet grille being located such that an upper end portion of the outlet grille is in contact with the recession, the front panel and the upper end portion of the outlet grille holding the roof part such that the front panel and the upper end portion of the outlet grille sandwich the roof part, the outdoor unit being characterised by the casing having a top plate provided on top of the front panel, the top plate having a top-plate extension portion that extends further forward than a tail end of the main roof portion and covering a portion of the roof part that is in the vicinity of the front panel, the top-plate extension portion having a flange portion that protrudes downward, a top face of the main roof portion and a lower edge (3 1a) of the flange portion being in contact with each other in a state in which the outlet grille is fixed to the casing.