In a reinforcing structure for reinforcing a vertical frame member made of aluminum material and constituting a sash frame by attaching thereto a reinforcing member made of steel, the reinforcing member is an elongated member composed of an indoor side piece, an outdoor side piece and a connection piece. The vertical frame member comprises an indoor side vertical plate member, an outdoor side vertical plate member and a connection plate member. The indoor side vertical plate member has an end portion which is bent to form an indoor side recessed portion continuously extending in the longitudinal direction thereof, and the outdoor side vertical plate member has an end portion which is bent to form an outdoor side recessed portion continuously extending in the longitudinal direction thereof. The reinforcing member is attached to the vertical frame member in the manner that the indoor side, outdoor side and connection pieces of the reinforcing member respectively face the indoor side vertical, outdoor side vertical and connection plate members of the vertical frame member, and that end portions of the indoor side and outdoor side pieces of the reinforcing member are respectively inserted in the indoor side and outdoor side recessed portions of the vertical frame member.