An air conditioner indoor unit comprising: a housing; an impeller arranged in the housing and forming a cross flow fan; a rear side plate arranged downstream the impeller and forming a rear side of a diffused air path; a front side plate forming a front side of the diffused air path and including a first air outlet surface, a second air outlet surface and a third air outlet surface; the first air outlet surface arranged near to the impeller and having a portion on a side of an air outlet of the diffused air path slanted in a direction away from a reference surface defined by the rear side plate; the second air outlet surface arranged next to the first air outlet surface on the side of the air outlet and having a portion on the side of the air outlet slanted in a direction away from the reference surface; and the third air outlet surface arranged next to the second air outlet surface on the side of the air outlet end and having a portion on the side of the air outlet slanted at 20.degree.-30.degree. in the direction away from the reference surface.