A container lid comprising a synthetic resin inside plug (2, 102, 202, 302) to be mounted on a mouth neck of a container, and a synthetic resin lid body (4, 204, 304) to be mounted on the inside plug (2, 102, 202,302), wherein the inside plug (2, 102, 202,302) includes a circular closing wall (6, 106), and a cylindrical mounting wall (8, 208, 308) connected to an outer peripheral edge of the closing wall (6, 106), and the mounting wall (8, 208, 308) is fitted to an outer peripheral surface of the mouth neck of the container, whereby when the inside plug (2, 102, 202, 302) is mounted on the mouth neck of the container the closing wall (6, 106) closes the mouth neck of the container, the lid body (4, 204, 304) includes a circular covering wall, and a cylindrical fitting wall (46, 246, 346) connected to an outer peripheral edge of the covering wall, and the fitting wall (46, 246, 346) is fitted to an outer peripheral surface (208a, 308a) of the mounting wall (8, 208, 308) of the inside plug (2, 102, 202, 302), whereby the lidbody (4, 204, 304) is rotatably mounted on the inside plug (2, 102, 202, 302), and the covering wall is located above the closing wall (6, 106) of the inside plug (2, 102, 202,302), at least one breakable thin-walled line is formed in the closing wall (6, 106) of the inside plug (2, 102, 202,302), the breakable thin-walled line (18, 118) has an outer arcuate portion (18b, 118b) extending from a breakage start end portion (18a, 118a), and an inner arcuate portion (18c, 118c) extending from the breakage start end portion (18a, 118a), and an arcuate deformation region (20, 120) is defined between the outer arcuate portion (18b, 118b) and the inner arcuate portion (18c, 118c), lockedmeans is annexed to the deformation region (20, 120), wherein locking means collaborating with the locked means is disposed on a lower surface of the covering wall, with the lid body (4, 204, 304) being mounted on the inside plug (2, 102, 202,302), the locking means is located upstream of the locked means as viewed in a predetermined direction; when the lid body (4, 204, 304) is rotated in the predetermined direction relative to the inside plug (2, 102, 202,302), the locking means is locked to the locked means; and when the lidbody (4, 204, 304) is rotated further in the predetermined direction relative to the inside plug (2, 102, 202, 302), a force is exerted on the deformation region (20, 120) via the locking means and the locked means, so that the breakable thin-walled line (18, 118) of the inside plug (2, 102, 202,302) is broken and the deformation region (20, 120) is deformed, whereby a pass-through opening (90, 190) is formed in the closing wall (6, 106) of the inside plug (2, 102, 202, 302), characterized in that at least one discharge opening is formed in the covering wall of the lid body (4, 204, 304), wherein two or three of the breakable thin-walled lines (18, 118) are formed at equal intervals in a circumferential direction in the closing wall (6, 106) of the insideplug (2, 102, 202, 302), and two or three of the deformation regions (20, 120) are defined at equal intervals in the circumferential direction, wherein the locked means is composed of a locked piece extending out upwardly from an upstream end part of the deformation region (20, 120), as viewed in the predetermined direction, and then extending inwardly in a radial direction, and each of the locking means is composed of a locking piece extending downwardly from the lower surface of the covering wall, and then extending in the predetermined direction.