Diffuser and bellmouth for an outdoor unit fan casing

An outdoor unit for a refrigeration cycle apparatus includes: a fan; a bellmouth that covers a periphery of the fan; a casing that internally houses the fan and the bellmouth so that airflow faces upward and that has a top end edge having a flat shape; and a diffuser surface disposed below a top end edge of the casing but above a top end of the bellmouth and on the outer side relative to the top end of the bellmouth in plan view, wherein the diffuser surface is inclined to extend outward away from the rotation axis of the fan as getting upward. The casing has a plurality of side surfaces that cover the sides of the bellmouth. At least one of the side surfaces has a recessed shape toward the bellmouth so that a gap with the top end edge of the bellmouth is reduced.