A synthetic resin cap comprises: a cap body having a top plate and a cylinder that is vertically lowered from the peripheral edge thereof; and a liner provided on a lower surface of the top plate. The liner has a flat plate; an inter-seal projection that contacts an inner-edge side of an opening end and has an outer-seal projection that contacts an outer-edge side of the opening end. An outer edge of the liner is spaced apart from the cylinder at an internal side and is formed to ensure a space between the outer edge and the cylinder. The outer diameter of the outer-seal projection is smaller than an outer diameter of the opening end.