A plastic cap comprising a top panel and a skirt hanging down from the peripheral edge of the top panel, the inner surface of the top panel forming an inner ring that forms a seal upon coming into intimate contact with the inner surface of the wall of the mouth of a container, and the inner surface of skirt forming a screw thread that comes into engagement with a screw thread formed on the outer surface of the wall of the mouth of the container, wherein a slender recessed portion is formed in a number of at least one in the inner surface of said inner ring extending upward from an end thereof. The cap features both excellent sealing and excellent venting.