A resin lid for a container body with an opening in an upper part, including a body part that closes the opening and has an air hole, and a flap that includes a base, a hinge on a first end of the base, and a projection provided on a second end side of the base and fitted in the air hole. The flap is swingable around the hinge between a first position in which the projection fits in the air hole and a second position in which the air hole is open. The body part and flap are integrally formed. When the flap travels from the second to the first position, a tip of the projection comes into contact with a circumferential edge of the opening of the air hole on a side opposite the hinge, and the projection fits in the air hole as the flap elastically deforms.