A container lid is composed of a synthetic resin inside plug to be mounted on the mouth of a container, and a synthetic resin lid to be mounted on the inside plug. A breakable thin-walled line formed in a closing wall has an outer arcuate portion extending from a breakage start end portion, and an inner arcuate portion extending from the breakage start end portion, and defines an arcuate deformation region. A locked element is annexed to the deformation region, and a locking element is in the lid. When the lid is rotated in a predetermined direction, the locking element is locked to the locked element. When the lid is rotated further in the predetermined direction, a force is exerted on the deformation region via the locking element and the locked element, so that the breakable thin-walled line of the inside plug is broken and the deformation region is deformed.