A seat for a baby carriage has a seat portion with a plate-type core member therein. The core member is divided into a plurality of plate-type members by at least one longitudinally extending parting line for allowing the baby carriage to be collapsed along the direction of its width. Adjacent ones of the plurality of plate-type members are hinged together with each other by hinge tapes on lower surface sides thereof. When the plate-type members define a uniform plane, end surfaces thereof are butted against each other so as to maintain the uniform plane even when a baby is seated on the seat portion. Thereby, the baby's buttocks are prevented from sinking down into the seat, and the baby is relieved of muscular strain.