A seat cushion of a seat, which can avoid creation of wrinkle or crease in an area of a trim cover element corresponding to a lateral side of the seat cushion, irrespective of a downward load applied to the seat cushion, thereby making it possible to maintain an outer appearance of the seat cushion in a good quality condition. In a surface of a recessed region of a padding forming a part of the seat cushion, defined flexible portions are formed to be flexed in an inward direction by a downward load applied thereto. Non-bonding areas are defined between the trim cover element and the flexible portions.